Consider adding Method::from_bytes_unchecked

[carllerche]

This function would bypass all checks on the input in favor of the caller guaranteeing that:

• The input only contains valid method characters
• The input is not a standard HTTP method.

This is a continuation of #8

[withoutboats]

Seems like the solution is to give this a name that makes it clear this should only be used to produce non-standard extension methods (as well as emphatic documentation of course). No idea what name that would be.. Method::extension_from_bytes_unchecked seems far too long but is all I have.

EDIT: maybe just Method::extension_unchecked?