Enhancement: Support gzip compression

[rickb777]

The Salesforce API allows gzip compression on either request entities or response entities (or both). It's not to hard to support this by using the necessary Content-Encoding and Accept-Encoding headers and then passing the json marshalling through compress/gzip functions.

The only method affected is ForceApi.request, I think.

[rickb777]

BTW in my experience, low compression levels are better than the defaults for this kind of use case; level 4 is used in the suggested code here. This amends ForceApi.request:

	// Build body
	var body io.Reader
	if payload != nil {
		buf := &bytes.Buffer{}

		gbuf, err := gzip.NewWriterLevel(buf, 4)
		if err != nil {
			return fmt.Errorf("Error compressing encoded payload: %v", err)
		}

		err = json.NewEncoder(gbuf).Encode(payload)
		if err != nil {
			return fmt.Errorf("Error marshaling encoded payload: %v", err)
		}

		body = buf
	}

	// Build Request
	req, err := http.NewRequest(method, uri.String(), body)
	if err != nil {
		return fmt.Errorf("Error creating %v request: %v", method, err)
	}

	// Add Headers
	req.Header.Set("User-Agent", userAgent)
	req.Header.Set("Content-Type", contentType)
	req.Header.Set("Content-Encoding", "gzip")
	req.Header.Set("Accept-Encoding", "gzip")
	req.Header.Set("Accept", responseType)
	req.Header.Set("Authorization", fmt.Sprintf("%v %v", "Bearer", forceApi.oauth.AccessToken))

	// Send
	forceApi.traceRequest(req)
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		return fmt.Errorf("Error sending %v request: %v", method, err)
	}
	defer resp.Body.Close()
	forceApi.traceResponse(resp)

	// Sometimes the force API returns no body, we should catch this early
	if resp.StatusCode == http.StatusNoContent {
		return nil
	}

	body = resp.Body
	if resp.Header.Get("Content-Encoding") == "gzip" {
		body, err = gzip.NewReader(resp.Body)
		if err != nil {
			return fmt.Errorf("Error decompressing %v response: %v", method, err)
		}
	}
	respBytes, err := ioutil.ReadAll(body)
	...  etc

The new bits of code would probably be best in new helper methods instead of inline.

[rickb777]

I would have made a PR but when I checked out the code I got many test failures.

[rickb777]

BTW the suggested code above uses the standard encoding/json package instead of the forcejson package. I'm not clear why forcejson exists, but if it's really needed it should have NewEncoder and NewDecoder functions like the standard library does.