internal/http/request.go: replace GetReason() with GetReasonFromBody()

Update tests to inject the required JSON error body with reason.

internal/http/request.go

@@ -66,7 +66,11 @@ func Request(method string, url string, contentType string, r io.Reader, w io.Wr
 	defer res.Body.Close()
 
 	if res.StatusCode < 200 || res.StatusCode >= 300 {
-		return errors.New(GetReason(res))
+		reason, err := GetReasonFromBody(res)
+		if err != nil {
+			return err
+		}
+		return errors.New(reason)
 	}
 
 	if w == nil {

internal/http/request_test.go

@@ -72,7 +72,9 @@ func TestReturnHTTPErrorAsIs(t *testing.T) {
 
 func TestReturnHTTPReasonAsErrorWhenUnsuccessful(t *testing.T) {
 	defer gock.Off()
-	gock.New("http://localhost").Reply(http.StatusTeapot)
+	gock.New("http://localhost").
+		Reply(http.StatusTeapot).
+		BodyString(`{"reason": "I'm a teapot"}`)
 
 	err := Request("GET", "http://localhost", "", nil, nil)
 	if err == nil || err.Error() != http.StatusText(http.StatusTeapot) {