Make working with $data keys case-insensitive. Closes #3 Closes #4

Make Brq_signature data key detection case-insensitive. The key was included in signature calculation and comparison when its case was different than 'Brq_signature'.
Make all $data keys in CompleteRequestPurchaseResponse upper case and change getters accordingly. Buckaroo may send $data keys in upper, mixed or lower case. In neither of those cases do they send the correct case.