POST methods do not send JSON encoded data

[hans2520]

Some DBT API POST methods, such as triggerRun, accept body JSON data structures as parameters (i.e. steps_override).

_post method of pydbtcloud does not offer a way to send the body with JSON-encoding -- as such, it is very difficult to properly use the DBT API for methods that utilize such parameters.

[hans2520]

I recommend the pydbtcloud _post method be modified to accept a headers parameter, which can be used by the caller to add a Content-Type: application/json header, which will then allow proper JSON data to be sent to DBT API.

I'll submit a PR for this.