ParseForm parses the raw query from the URL and updates r.Form.

For POST or PUT requests, it also parses the request body as a form and put the results into both r.PostForm and r.Form. POST and PUT body parameters take precedence over URL query string values in r.Form.

If the request Body's size has not already been limited by MaxBytesReader, the size is capped at 10MB.

ParseMultipartForm calls ParseForm automatically. It is idempotent.