Don't alter query string when submitting form by POST.

components/script/dom/htmlformelement.rs

@@ -459,15 +459,16 @@ impl HTMLFormElement {
                     .headers
                     .typed_insert(ContentType::from(mime::APPLICATION_WWW_FORM_URLENCODED));
 
-                self.set_encoding_override(load_data.url.as_mut_url().query_pairs_mut())
+                let mut url = load_data.url.clone();
+                self.set_encoding_override(url.as_mut_url().query_pairs_mut())
                     .clear()
                     .extend_pairs(
                         form_data
                             .into_iter()
                             .map(|field| (field.name.clone(), field.replace_value(charset))),
                     );
 
-                load_data.url.query().unwrap_or("").to_string().into_bytes()
+                url.query().unwrap_or("").to_string().into_bytes()
             },
             FormEncType::FormDataEncoded => {
                 let mime: Mime = format!("multipart/form-data; boundary={}", boundary)

tests/wpt/metadata/MANIFEST.json

    "support"
   ], 
   "common/form-submission.py": [
-   "467875453c9dc64aac51add3f4a617d941820972", 
+   "b296ac44c57c658ca9e3189e9f79b0f80677acba", 
    "support"
   ], 
   "common/get-host-info.sub.js": [
    "testharness"
   ], 
   "html/semantics/forms/form-submission-0/submit-entity-body.html": [
-   "8363f39ff32cd165a28086b3c594b55a79ced8e5", 
+   "a52bb0dcd2080eabb44ab1ddca379da6a5e5ea9a", 
    "testharness"
   ], 
   "html/semantics/forms/form-submission-0/submit-file.sub.html": [

tests/wpt/web-platform-tests/common/form-submission.py

@@ -6,5 +6,7 @@ def main(request, response):
     else:
         result = request.POST.first('foo') == 'bar'
 
+    result = result and request.url_parts.query == 'query=1'
+
     return ([("Content-Type", "text/plain")],
             "OK" if result else "FAIL")

tests/wpt/web-platform-tests/html/semantics/forms/form-submission-0/submit-entity-body.html

@@ -96,7 +96,7 @@
   var testframe = document.getElementById("testframe");
   var testdocument = testframe.contentWindow.document;
   testdocument.body.innerHTML =
-    "<form id=testform method=post action=\"form-submission.py\" enctype=\"" + test_obj.enctype + "\">" +
+    "<form id=testform method=post action=\"form-submission.py?query=1\" enctype=\"" + test_obj.enctype + "\">" +
     test_obj.input +
     test_obj.submitelement +
     "</form>";