Using the Authorization header on the get_token method is only required when using a legacy version of KeyRock

[aarranz]

Current implementation provides an Authorization header with the client id and client secret using base64 encoding.

We have tested the following services with the following results:

• KeyRock v5 and v6. Needs the Authorization header when requesting access tokens.
• Github and KeyRock v7 does not required the Authorization header but works if you use it (problably this header get ignored).
• WSO2-IS refuses to give a token if you use this header as the credentials are sent on the body, so they complain about repeated credentials.

This PR change the code so this header is only used when connecting to a legacy version of the IdM. Should fix #22.

[coveralls]

Coverage increased (+0.007%) to 99.559% when pulling 9560fd68f1f2495c92a20458dd14adf1f50f6fe6 on aarranz:fix/wso2-get-token into 3749d7d7349bffac52a64c6ee9ae766d811d5b1c on conwetlab:master.