http: expose supported methods

Expose the list of supported HTTP methods as a property on the 'http'
module object.

Fixes #6422.
http: remove MethodToString()

The list of supported HTTP methods is available in JS land now so there
is no longer any need to pass a stringified version of the method to the
parser callback, it can look up the method name for itself.

Saves a call to v8::Eternal::Get() in the common case and a costly
v8::String::NewFromOneByte() in the uncommon case.