Merge pull request #147 from perosb/getExtFix

Dont throw exeption if querystring is null

Former-commit-id: ff55b7b75b7e308c109e0b43f7afd0cb7076f3a9
Former-commit-id: 1fa27c75f1ed4cbfd348a5792b07893bfc6b24cb
Former-commit-id: 85172fcad62216d174113f44cddf0f933d1db24e

src/ImageProcessor.Web/Helpers/ImageHelpers.cs

@@ -78,7 +78,12 @@ namespace ImageProcessor.Web.Helpers
             {
                 // Test against the path minus the querystring so any other
                 // processors don't interere.
-                string trimmed = fullPath.Replace(queryString, string.Empty);
+                string trimmed = fullPath;
+                if (queryString != null)
+                {
+                    trimmed = trimmed.Replace(queryString, string.Empty);
+                }
+                
                 match = FormatRegex.Match(trimmed);
             }