HttpAdapterSpray doesn't support MessageChunk

[terrhorn]

Extractions of any significant size return chunked response bodies which causes KeenClient to throw an HttpResponse to MessageChunk type conversion error.

Workaround is to use HttpAdapterDispatch.

[ches]

Chunked responses were comprehensively improved in akka-http, so this ought to be something that can be addressed when upgrading the library from the old versions of Spray, although it might warrant some reworking of the API.