add toUpperCase when check for method

[yuzurihaaa]

WHY

Current if there's request made into http.Client() with method "get", it will throw error.

ClientException: Invalid argument(s): Unsupported method: get, uri=...
#0      RhttpCompatibleClient.send (package:rhttp/src/client/compatible_client.dart:70:16)

It used to work because in dart-lang, it was later on called with toUpperCased https://github.com/dart-lang/sdk/blob/d878cfbf20375befa09f9bf85f0ba2b87b319427/sdk/lib/_http/http_impl.dart#L2323

WHAT

1. Add toUpperCase for method.

[Tienisto]

This is already fixed by another PR: #57