Fix/param interpolation

Previously params were merged into query on `GET/HEAD` and then the query values were merged into the URL for all request methods.

This was not the behavior described in the README.  With this change, params are merged into the URL for all request methods and then the remainder of params are either placed in the query for `GET/HEAD` or the request body for `POST/PUT/PATCH`.

This will probably break existing clients, but it makes the code more consistent with the documented behavior and allows us to use interpolation in `POST/PUT/PATCH` urls