Extract request from multiple sources in render api

[AnderEnder]

Render API request can use GET(url_encoded params) and POST(form and json). It is possible to construct data query from url + from json/form payload for POST requests.

• provide the rules how to merge information from url_encoded with payload type
• provide validation of the request data of merged structure
• covers with tests the expected behaviours

Docs:

• https://graphite-api.readthedocs.io/en/latest/api.html#the-render-api-render
• https://graphite.readthedocs.io/en/latest/render_api.html