use uppercase "type" for Proxy-Authorization header (#19273)

Some servers will reject authorization requests with a lowercase "basic" type. Changing to "Basic" seems to solve these issues.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Proxy-Authorization
(cherry picked from commit b812431)

lib/pure/httpclient.nim

@@ -523,7 +523,7 @@ proc generateHeaders(requestUrl: Uri, httpMethod: HttpMethod, headers: HttpHeade
   # Proxy auth header.
   if not proxy.isNil and proxy.auth != "":
     let auth = base64.encode(proxy.auth)
-    add(result, "Proxy-Authorization: basic " & auth & httpNewLine)
+    add(result, "Proxy-Authorization: Basic " & auth & httpNewLine)
 
   for key, val in headers:
     add(result, key & ": " & val & httpNewLine)