Make use of Faraday::GetMethodOverride optional

[rhymes]

I made this middleware optional, so that it can be deactivated.

It still defaults to true because that's why previous users of the library expect. We should consider switching it to false at some point.

Closes #210

[coveralls]

Coverage increased (+0.002%) to 98.07% when pulling cb434c2 on optional-method-override into 9b79ec7 on master.

[coveralls]

Coverage increased (+0.004%) to 98.073% when pulling 087f7c3 on optional-method-override into 9b79ec7 on master.

[coveralls]

Coverage increased (+0.004%) to 98.07% when pulling 4c1f6c1 on optional-method-override into da81f45 on master.

[coveralls]

Coverage increased (+0.004%) to 98.07% when pulling 2ea35f6 on optional-method-override into 5cd24ca on master.