GET Submissions: Do not merge into [action] (#464)

Follow-up to [#461][]

While the changes made in #461 added coverage for `<form method="post"
action="...">` and `<form>` submissions, it didn't cover `<form
action="/?encoded=value">` submissions with search parameters encoded
into the `[action]` attribute.

Without Turbo or any other JavaScript, submitting a form with an
`[action]` value that contains query parameters requests that `[action]`
path without any key-value pairs (that is, with `?` as its the value of
`URL.search`):

```html
<form action="/src/tests/fixtures/form.html?sort=asc">
  <!-- clicking the button navigates to `/src/tests/fixtures/form?` -->
  <button>Submit</button>
</form>
```

This commit modifies the `FetchRequest` to always invoke the
module-private `mergeFormDataEntries()` function when constructing `GET`
requests, and modifies the `mergeFormDataEntries()` to completely
discard the `[action]` URL's search parameters before merging-in the
`FormData` key-value pairs.

[#461]: #461

src/http/fetch_request.ts

@@ -57,7 +57,7 @@ export class FetchRequest {
     this.method = method
     this.headers = this.defaultHeaders
     this.body = body
-    this.url = this.isIdempotent && this.entries.length ?
+    this.url = this.isIdempotent ?
       mergeFormDataEntries(new URL(location.href), this.entries) :
       location
     this.target = target
@@ -152,25 +152,15 @@ export class FetchRequest {
 }
 
 function mergeFormDataEntries(url: URL, entries: [string, FormDataEntryValue][]): URL {
-  const currentSearchParams = new URLSearchParams(url.search)
-  deleteAll(url.searchParams)
+  const searchParams = new URLSearchParams
 
   for (const [ name, value ] of entries) {
     if (value instanceof File) continue
 
-    if (currentSearchParams.has(name)) {
-      currentSearchParams.delete(name)
-      url.searchParams.set(name, value)
-    } else {
-      url.searchParams.append(name, value)
-    }
+    searchParams.append(name, value)
   }
 
-  return url
-}
+  url.search = searchParams.toString()
 
-function deleteAll(searchParams: URLSearchParams) {
-  for (const name of searchParams.keys()) {
-    searchParams.delete(name)
-  }
+  return url
 }

src/tests/fixtures/form.html

@@ -29,10 +29,13 @@ <h1>Form</h1>
       <form>
         <button id="form-action-none-q-a" name="q" value="a">Submit ?q=a to form:not([action])</button>
       </form>
-      <form action="/src/tests/fixtures/form.html?sort=asc">
-        <button id="form-action-self-submit">Submit form[action]</button>
+      <form action="/src/tests/fixtures/form.html">
+        <button id="form-action-self-sort" name="sort" value="asc">Submit ?sort=asc to form[action]</button>
         <button id="form-action-self-q-b" name="q" value="b">Submit ?q=b to form[action]</button>
       </form>
+      <form action="/src/tests/fixtures/form.html?q=c">
+        <button id="form-action-self-submit">Submit form[action]</button>
+      </form>
       <form action="/__turbo/redirect?path=%2Fsrc%2Ftests%2Ffixtures%2Fform.html%3Fsort%3Dasc" method="post">
         <button id="form-action-post-redirect-self-q-b" name="q" value="b">POST q=b to form[action]</button>
       </form>

src/tests/functional/form_submission_tests.ts

@@ -136,7 +136,7 @@ export class FormSubmissionTests extends TurboDriveTestCase {
   }
 
   async "test standard GET form submission does not incorporate the current page's URLSearchParams values into the submission"() {
-    await this.clickSelector("#form-action-self-submit")
+    await this.clickSelector("#form-action-self-sort")
     await this.nextBody
 
     this.assert.equal(await this.pathname, "/src/tests/fixtures/form.html")
@@ -150,7 +150,7 @@ export class FormSubmissionTests extends TurboDriveTestCase {
   }
 
   async "test standard GET form submission does not merge values into the [action] attribute"() {
-    await this.clickSelector("#form-action-self-submit")
+    await this.clickSelector("#form-action-self-sort")
     await this.nextBody
 
     this.assert.equal(await this.pathname, "/src/tests/fixtures/form.html")
@@ -163,6 +163,14 @@ export class FormSubmissionTests extends TurboDriveTestCase {
     this.assert.equal(await this.search, "?q=b", "navigates without omitted keys")
   }
 
+  async "test standard GET form submission omits the [action] value's URLSearchParams from the submission"() {
+    await this.clickSelector("#form-action-self-submit")
+    await this.nextBody
+
+    this.assert.equal(await this.pathname, "/src/tests/fixtures/form.html")
+    this.assert.equal(await this.search, "")
+  }
+
   async "test standard GET form submission toggles submitter [disabled] attribute"() {
     await this.clickSelector("#standard-get-form-submit")