Don't duplicate headers

[tlmii]

Fixes #237.

There were actually two separate things going on:

1. As pointed out by POST/PATCH with body adds custom headers twice #237, almost all headers were duplicated if you were doing anything with a request body (because headers were being set for both the request and the content).
2. The fix in update to fix duplicated ContentType with Test #277 showed the other part of it - that Content-Type specifically was always being duplicated (with or without a request body) if it was added with set header or the --header argument, because it was special-cased and set early before all those were added in bulk.

This PR addresses both issues, and supersedes #277, which I'll close with an explanation once this goes in.

[bzhu94]

Looks good!