LibWeb: Make about:blank load correctly

- Don't treat an empty `about:blank` resource as an error.
- Give `about:` urls a content-type so `FrameLoader::parse_document()`
  won't reject them.

Userland/Libraries/LibWeb/Loader/FrameLoader.cpp

@@ -331,7 +331,7 @@ void FrameLoader::resource_did_load()
     }
     m_redirects_count = 0;
 
-    if (!resource()->has_encoded_data()) {
+    if (!resource()->has_encoded_data() && url.to_string() != "about:blank") {
         load_error_page(url, "No data");
         return;
     }