Avoid null exception (#1619)

Return better error info for client/server enum mismatch.

Author: sicklittlemonkey

src/RestSharp/Extensions/ReflectionExtensions.cs

@@ -93,6 +93,8 @@ public static bool IsSubclassOfRawGeneric(this Type toCheck, Type generic)
             if (enumValueAsUnderlyingType != null && Enum.IsDefined(type, enumValueAsUnderlyingType))
                 ret = (Enum) Enum.ToObject(type, enumValueAsUnderlyingType);
 
+            if (ret == null)
+                throw new ArgumentOutOfRangeException(value + " undefined for " + type);
             return ret;
         }
     }