handle json post requests

[shmargum]

This adds basic support for sending authenticated JSON post requests.
Resolves #862
Also resolves #869

Example usage:

    Twitter::REST::Request.new(
      client, # Twitter::REST::Client instance
      :json_post, # <-- this is important
      '/1.1/the-endpoint.json',
      {
        some_key: "some value",
      }
    ).perform

[shmargum]

Just leaving an explanation behind this PR:
We need to be able to send authenticated requests with a JSON (string) payload, with the correct header application/json

For the correct oauth headers, we must not include the JSON payload in the signature.
In order to get the underlying HTTP client to send the proper application/json header, we need to send the options under the key :json : https://github.com/httprb/http/blob/d09394c5dcf9cc28b4858a78dc071091bcce4b54/lib/http/client.rb#L176

[shmargum]

@sferik hoping you can have a look at this

[bobber205]

I can confirm this allows me to create welcome messages easily!

[clarkbab]

@shmargum @sferik Any movement on getting this merged? I believe that the old DM endpoints will cease to work on June 19th. Not long!

[bobber205]

FWIW the pull request is missing a put equivalent (I added a PR for it) shmargum#1

[sferik]

@shmargum I recently merged #927. Does this solve your problem?
If so, can this PR be closed?