Add string-based accessors for Request method

[harmony7]

Method::get_method()/set_method()/with_method() only support the 9 standard HTTP verbs via the Method enum, which cannot represent nonstandard-but-common verbs like PURGE.

This PR adds get_method_str(), Request::create(), and string overloads of set_method()/with_method() that accept any valid HTTP method token, backed by the fastly crate's existing impl ToMethod support for strings. Invalid tokens are
reported via a new FastlyErrorCode::InvalidMethod rather than panicking.

Fixes #88

NOTE: get_method() will panic if the method is not one of the nine. The right way to fix this is probably to change that function's signature to return expected<>, but that would be breaking.