fix 204 No Content handling so that API does not croak on missing data

GitLab/API.pm

@@ -229,12 +229,17 @@ sub clean_data {
         croak $response->message;
     }
 
-    # all responses from GitLab should be JSON data
-    if ($response->header("Content-Type") !~ m!^application/json!) {
-        croak "GitLab sent '", $response->header("Content-Type"), "' instead of 'application/json'";
-    }
+    # no data should be present if we got 204 No Content
+    return $response
+        if $response->code == HTTP_NO_CONTENT;
+
+    # otherwise we should have gotten application/json response
+    croak "GitLab sent '", $response->header("Content-Type"), "' instead of 'application/json'"
+        if $response->header("Content-Type") !~ m!^application/json!;
 
-    return ($response, undef) unless $response->is_success;
+    # don't process data on error
+    return ($response, undef)
+        unless $response->is_success;
 
     my $data = $response->decoded_content;
     utf8::decode($data);