POST with `:body` instead of `:query`

I updated the `HTTParty.post` to use the `:body` parameter instead of `:query`. Submitting data through the POST `:body` allows us to get around data-length limitations associated with using `:query` string parameters.

Sendgrid Article: [HTTP POSTing The Right Way](https://sendgrid.zendesk.com/hc/en-us/articles/200181828-HTTP-POSTing-The-Right-Way)

I also updated the tests to support this change. Unfortunately [WebMock](https://github.com/chrisk/fakeweb#known-issues) doesn't allow to specific request bodies.