Added interface for custom loggers

This interface should allow to plug-in custom loggers
more easily.

client.go

@@ -183,18 +183,24 @@ func NewRequest(method, url string, rawBody interface{}) (*Request, error) {
 	return &Request{body, httpReq}, nil
 }
 
+// Logger interface allows to use other loggers than
+// standard log.Logger.
+type Logger interface {
+	Printf(string, ...interface{})
+}
+
 // RequestLogHook allows a function to run before each retry. The HTTP
 // request which will be made, and the retry number (0 for the initial
 // request) are available to users. The internal logger is exposed to
 // consumers.
-type RequestLogHook func(*log.Logger, *http.Request, int)
+type RequestLogHook func(Logger, *http.Request, int)
 
 // ResponseLogHook is like RequestLogHook, but allows running a function
 // on each HTTP response. This function will be invoked at the end of
 // every HTTP request executed, regardless of whether a subsequent retry
 // needs to be performed or not. If the response body is read or closed
 // from this method, this will affect the response returned from Do().
-type ResponseLogHook func(*log.Logger, *http.Response)
+type ResponseLogHook func(Logger, *http.Response)
 
 // CheckRetry specifies a policy for handling retries. It is called
 // following each request with the response and error values returned by
@@ -221,7 +227,7 @@ type ErrorHandler func(resp *http.Response, err error, numTries int) (*http.Resp
 // like automatic retries to tolerate minor outages.
 type Client struct {
 	HTTPClient *http.Client // Internal HTTP client.
-	Logger     *log.Logger  // Customer logger instance.
+	Logger     Logger       // Customer logger instance.
 
 	RetryWaitMin time.Duration // Minimum time to wait
 	RetryWaitMax time.Duration // Maximum time to wait

client_test.go

@@ -110,7 +110,7 @@ func testClient_Do(t *testing.T, body interface{}) {
 	client.RetryWaitMin = 10 * time.Millisecond
 	client.RetryWaitMax = 50 * time.Millisecond
 	client.RetryMax = 50
-	client.RequestLogHook = func(logger *log.Logger, req *http.Request, retryNumber int) {
+	client.RequestLogHook = func(logger Logger, req *http.Request, retryNumber int) {
 		retryCount = retryNumber
 
 		if logger != client.Logger {
@@ -275,7 +275,7 @@ func TestClient_RequestLogHook(t *testing.T) {
 	testURIPath := "/foo/bar"
 
 	client := NewClient()
-	client.RequestLogHook = func(logger *log.Logger, req *http.Request, retry int) {
+	client.RequestLogHook = func(logger Logger, req *http.Request, retry int) {
 		retries = retry
 
 		if logger != client.Logger {
@@ -325,7 +325,7 @@ func TestClient_ResponseLogHook(t *testing.T) {
 	client.RetryWaitMin = 10 * time.Millisecond
 	client.RetryWaitMax = 10 * time.Millisecond
 	client.RetryMax = 15
-	client.ResponseLogHook = func(logger *log.Logger, resp *http.Response) {
+	client.ResponseLogHook = func(logger Logger, resp *http.Response) {
 		if resp.StatusCode == 200 {
 			// Log something when we get a 200
 			logger.Printf("test_log_pass")