HTTP: add content-type: application/json header
Merge pull request #45 from jpfuentes2/jf_use_json_content_type

command/agent: add content-type: application/json header to HTTP API
Update CHANGELOG
Ensure we write json header before writing bytes

In net/http once we've issued a Write() the response is sent
over the wire including the header! The tests didn't catch
this because I used a net/http/httptest.RequestRecorder
which doesn't follow those semantics.