Setup an interface for writing http responses based on the Content-Type header

[jsternberg]

Only application/json is supported right now, but this opens up the
easier possibility of additional content types to be returned from the
server.

[mention-bot]

By analyzing the blame information on this pull request, we identified @e-dard, @gunnaraasen and @joelegasse to be potential reviewers

[jsternberg]

There's still more work involved with completely integrating this. Errors are still always sent back as JSON instead of the proper content type.

[jsternberg]

I think this should work correctly now. Now other handlers, like authentication, go through the custom ResponseWriter that controls formatting the output. The package continues to pass golint.

The only caveat is that a minor change has been made to the output that shouldn't matter, but is worth noting. Queries without chunking previously would not end in a newline. Now queries without chunking end in a newline to be consistent with everybody else. That adds one extra byte to be returned. If we need to keep the old behavior, it's possible, but requires much hackier code.

[corylanou]

+1 looks good.