Spec compatibility:

• 1st argument to Request constructor must be string or Request
• Always construct a new Request instance in fetch()
• Always construct a new Headers instance in Response
• Avoid consuming body when cloning
• Add support for TypedArray/DataView as POST body
• ArrayBuffer, TypedArray, and Dataview bodies can now be accessed through any of the arrayBuffer(), text(), or blob() methods
• Default Response status is 200 OK

Other fixes:

• Make X-Request-URL header case-insensitive
• Allow reusing the same GET Request instance multiple times
• Rework parsing of raw response HTTP headers
• Attach FileReader event handlers before calling its read* method