Correct bilean reraising of exception

When an exception was caught and rethrown, it should call 'raise'
without any arguments because it shows the place where an exception
occured initially instead of place where the exception re-raised

Change-Id: Iee0d0c14c77e4ae2dbd0c3661eb7e3959a244290

bilean/common/schema.py

@@ -228,8 +228,8 @@ class String(Schema):
     def resolve(self, value):
         try:
             return str(value)
-        except (TypeError, ValueError) as ex:
-            raise ex
+        except (TypeError, ValueError):
+            raise
 
     def validate(self, value, context=None):
         if not isinstance(value, six.string_types):