Unifies how we fetch the client_id from the request and allows us to throw a more appropriate exception when the client_id parameter is missing.
Improves the test method for this validation by checking the culpable method in this particular case. The test was missing this by calling the wrong method.
According to [RFC7636#section-4.3](https://tools.ietf.org/html/rfc7636#section-4.3):
If the "code_challenge_method" from Section 4.3 was "S256", the
received "code_verifier" is hashed by SHA-256, base64url-encoded, and
then compared to the "code_challenge", i.e.:
BASE64URL-ENCODE(SHA256(ASCII(code_verifier))) == code_challenge
So, the hash must be done before the base64_encode.
The tests are modified to use example data from the
[RFC7636#appendix-B](https://tools.ietf.org/html/rfc7636#appendix-B).
The current implementation of PKCE does not follow the specification
correctly regarding the encoding of the code verifier. This patch
correctly encodes the hash of the code verifier according to
Appenix A of RFC 7636.
Should allow for better compatibility with server implementations that aren't sitting on top of a standard SAPI (e.g. persistent web servers building a PSR-7 compatible request from a socket-received message).
One catch here is that I've seen Apache hijack the HTTP Authorization header in the past, though that would probably impact the other aspects of the server just as much as it would this, so I think that risk is manageable.
Added tests to cover all paths through the new code, so the AbstractGrant type still has 100% coverage :)
Did notice that, as of the latest versions of PHPUnit, the mock creation method is deprecated. Maybe that needs to be updated? Haven't checked to see whether the replacements are PHPUnit 4.8 compatible though, so maybe they need to stay in order to test on older PHP versions?
