Add an example for POST method HTTP Provider

[agithony]

Per the documentation for HTTP Provider:

The method property MUST be a JSON string, such as "POST", as per RFC 7231. It defaults to "GET".

When provided, the arguments property MUST be a JSON object which properties are parameters of the HTTP request.

When method is "GET", the arguments are mapped as a query-string of the URL. Otherwise, the arguments are passed as the request body’s data and the encoding depends on the "Content-Type" provided in the headers object.

1. Can you provide an example using an POST method with some included arguments passed into the argument body (request body)?
2. Can you go into more details about the various encoding based on the "Content-Type" in the header?