Specify headers

[seattlevine]

Two separate but related changes:

• Allow non-json bodies.
• Allow arbitrary headers.

Use cases are:

• Allow for commands to be issued in a transaction. This requires setting the X-F5-REST-Coordination-Id header
• Allow calling the file uploader. This requires setting the Content-Range header and using a non-json body.

Note that this does not include pull request #4 so if these are both accepted, some manual merging is likely required.