PostForm issues a POST to the specified URL, with data's keys and values URL-encoded as the request body.

The Content-Type header is set to application/x-www-form-urlencoded. To set other headers, use NewRequest and DefaultClient.Do.

When err is nil, resp always contains a non-nil resp.Body. Caller should close resp.Body when done reading from it.

PostForm is a wrapper around DefaultClient.PostForm.