bunq header handling as enum (bunq/sdk_java#93)

[tubbynl]

i extracted magic constants for headers into a BunqHeader enum, added some junit tests on it

behaviour is as-is, but some tests are not really runnable locally

i'm using the bunq api currently in a spring-boot setup using rest-template, most of the gson/models i can use for deserializing the responses and in order to mimic the header handling i needed to copy some of the header names into my project.

i extracted that into an BunqHeader enum which you see here, if this get's into the sdk_java project i can remove this from my project :)

[tubbynl]

need to look in to this, headers are probably not-yet as they were before, i'll extract a testcase of header content in the correct case

[tubbynl]

see #100 close as-you-wish

[OGKevin]

@patrickdw1991 please 👀