Use application/x-www-form-urlencoded format for URL query strings

The main motivation here is to make sure we don't %-escape incoming pluses, but rather interpret them as spaces.

c++/src/kj/compat/url-test.c++

@@ -132,6 +132,14 @@ KJ_TEST("parse / stringify URL") {
     KJ_EXPECT(KJ_ASSERT_NONNULL(url.fragment) == "garply");
   }
 
+  {
+    auto url = parseAndCheck("https://capnproto.org/foo?bar%20baz=qux+quux",
+                             "https://capnproto.org/foo?bar+baz=qux+quux");
+    KJ_ASSERT(url.query.size() == 1);
+    KJ_EXPECT(url.query[0].name == "bar baz");
+    KJ_EXPECT(url.query[0].value == "qux quux");
+  }
+
   {
     auto url = parseAndCheck("https://capnproto.org/foo/bar#garply");
     KJ_EXPECT(url.scheme == "https");
@@ -232,7 +240,7 @@ KJ_TEST("URL percent encoding") {
 
   parseAndCheck(
       "https://b b: bcd@capnproto.org/f o?b r=b z#q x",
-      "https://b%20b:%20bcd@capnproto.org/f%20o?b%20r=b%20z#q%20x");
+      "https://b%20b:%20bcd@capnproto.org/f%20o?b+r=b+z#q%20x");
 }
 
 KJ_TEST("URL relative paths") {
@@ -340,6 +348,9 @@ KJ_TEST("parse relative URL") {
   parseAndCheckRelative("https://capnproto.org/foo/bar?baz=qux#corge",
                         "?grault",
                         "https://capnproto.org/foo/bar?grault");
+  parseAndCheckRelative("https://capnproto.org/foo/bar?baz=qux#corge",
+                        "?grault+garply=waldo",
+                        "https://capnproto.org/foo/bar?grault+garply=waldo");
   parseAndCheckRelative("https://capnproto.org/foo/bar?baz=qux#corge",
                         "grault",
                         "https://capnproto.org/foo/grault");

c++/src/kj/compat/url.c++

@@ -88,6 +88,12 @@ String percentDecode(ArrayPtr<const char> text, bool& hadErrors) {
   return kj::mv(result);
 }
 
+String percentDecodeQuery(ArrayPtr<const char> text, bool& hadErrors) {
+  auto result = decodeWwwForm(text);
+  if (result.hadErrors) hadErrors = true;
+  return kj::mv(result);
+}
+
 }  // namespace
 
 Url::~Url() noexcept(false) {}
@@ -195,9 +201,10 @@ Maybe<Url> Url::tryParse(StringPtr text, Context context) {
 
       if (part.size() > 0) {
         KJ_IF_MAYBE(key, trySplit(part, '=')) {
-          result.query.add(QueryParam { percentDecode(*key, err), percentDecode(part, err) });
+          result.query.add(QueryParam { percentDecodeQuery(*key, err),
+                                        percentDecodeQuery(part, err) });
         } else {
-          result.query.add(QueryParam { percentDecode(part, err), nullptr });
+          result.query.add(QueryParam { percentDecodeQuery(part, err), nullptr });
         }
       }
     } while (text.startsWith("&"));
@@ -331,9 +338,10 @@ Maybe<Url> Url::tryParseRelative(StringPtr text) const {
 
       if (part.size() > 0) {
         KJ_IF_MAYBE(key, trySplit(part, '=')) {
-          result.query.add(QueryParam { percentDecode(*key, err), percentDecode(part, err) });
+          result.query.add(QueryParam { percentDecodeQuery(*key, err),
+                                        percentDecodeQuery(part, err) });
         } else {
-          result.query.add(QueryParam { percentDecode(part, err), nullptr });
+          result.query.add(QueryParam { percentDecodeQuery(part, err), nullptr });
         }
       }
     } while (text.startsWith("&"));
@@ -407,10 +415,10 @@ String Url::toString(Context context) const {
   for (auto& param: query) {
     chars.add(first ? '?' : '&');
     first = false;
-    chars.addAll(encodeUriComponent(param.name));
+    chars.addAll(encodeWwwForm(param.name));
     if (param.value.size() > 0) {
       chars.add('=');
-      chars.addAll(encodeUriComponent(param.value));
+      chars.addAll(encodeWwwForm(param.value));
     }
   }