response.redirect: extract special-cased back, fixes #904 (#1115)

The simple solution to drop special-cased `'back'` in
`response.redirect`.
I can't label but should be labeled as version-major.

This PR **does not** make use of Symbol as purposed in #904.

**Edit**
If this solution is acceptable, a deprecation should be added to Koa 2
on `'back'` use.

---------

Co-authored-by: Kevin Peno <kjpeno@outlook.com>

__tests__/response/back.js

@@ -0,0 +1,32 @@
+'use strict'
+
+const assert = require('assert')
+const context = require('../../test-helpers/context')
+
+describe('ctx.back([alt])', () => {
+  it('should redirect to Referrer', () => {
+    const ctx = context()
+    ctx.req.headers.referrer = '/login'
+    ctx.back()
+    assert.equal(ctx.response.header.location, '/login')
+  })
+
+  it('should redirect to Referer', () => {
+    const ctx = context()
+    ctx.req.headers.referer = '/login'
+    ctx.back()
+    assert.equal(ctx.response.header.location, '/login')
+  })
+
+  it('should default to alt', () => {
+    const ctx = context()
+    ctx.back('/index.html')
+    assert.equal(ctx.response.header.location, '/index.html')
+  })
+
+  it('should default redirect to /', () => {
+    const ctx = context()
+    ctx.back()
+    assert.equal(ctx.response.header.location, '/')
+  })
+})

lib/context.js

@@ -204,6 +204,7 @@ delegate(proto, 'response')
   .method('set')
   .method('append')
   .method('flushHeaders')
+  .method('back')
   .access('status')
   .access('message')
   .access('body')

lib/response.js

@@ -18,6 +18,7 @@ const only = require('./only.js')
 const util = require('util')
 const encodeUrl = require('encodeurl')
 const Stream = require('stream')
+const deprecate = require('depd')('koa')
 
 /**
  * Prototype.
@@ -293,7 +294,11 @@ module.exports = {
 
   redirect (url, alt) {
     // location
-    if (url === 'back') url = this.ctx.get('Referrer') || alt || '/'
+    if (url === 'back') {
+      deprecate('Special-cased string "back" through redirect will be removed in v3, ' +
+        'consider migrating usage to ctx.back() instead.')
+      url = this.ctx.get('Referrer') || alt || '/'
+    }
     if (/^https?:\/\//i.test(url)) {
       // formatting url again avoid security escapes
       url = new URL(url).toString()
@@ -316,6 +321,24 @@ module.exports = {
     this.body = `Redirecting to ${url}.`
   },
 
+  /**
+   * Perform a special-cased "back" to provide Referrer support.
+   * When Referrer is not present, `alt` or "/" is used.
+   *
+   * Examples:
+   *
+   *    ctx.back()
+   *    ctx.back('/index.html')
+   *
+   * @param {String} [alt]
+   * @api public
+   */
+
+  back (alt) {
+    const url = this.ctx.get('Referrer') || alt || '/'
+    this.redirect(url)
+  },
+
   /**
    * Set Content-Disposition header to "attachment" with optional `filename`.
    *