Always pass a valid rounding value

Fixes #6015

Prevent an exception in `quantize()` when monkey-patching the decimal
library as cdecimal in Python 2 environments by always passing a valid
(not None) `rounding` value to `quantize()`.

rest_framework/fields.py

@@ -1135,11 +1135,12 @@ class DecimalField(Field):
             return value
 
         context = decimal.getcontext().copy()
+        rounding = context.rounding if self.rounding is None else self.rounding
         if self.max_digits is not None:
             context.prec = self.max_digits
         return value.quantize(
             decimal.Decimal('.1') ** self.decimal_places,
-            rounding=self.rounding,
+            rounding=rounding,
             context=context
         )