Fix mxit transport access token retrieval to always return a bytestring
Fix mxit transport send api url and scope config defaults to match what mxit's api expects
Merge branch 'feature/issue-891-fix-sending-for-mxit-transport' into develop
Avoid setting multipart stuff for non-multipart messages.
Merge branch 'develop' into feature/issue-872-use-smpp-esm-class-to-detect-dr