Fix phonenumber Django 1.6/Python 3 unicode bug

Django calls force_text on PhoneNumber instances when populating
a model form. Django 1.6's force_text implementation unfortunately
checks for __unicode__, which is defined in the base class.
As we're using the python_2_unicode_compatible decorator, in Python 2
__unicode__ is already overwritten by the decorator.
In Python 3, we can safely return __str__ because it returns unicode.

This was discovered by @itbabu in #1645.

src/oscar/core/phonenumber.py

@@ -68,6 +68,15 @@ def __str__(self):
             return self.format_as(fmt)
         return self.raw_input
 
+    def __unicode__(self):
+        # Django calls force_text on PhoneNumber instances when populating
+        # a model form. Django 1.6's force_text implementation unfortunately
+        # checks for __unicode__, which is defined in the base class.
+        # As we're using the python_2_unicode_compatible decorator, in Python 2
+        # __unicode__ is already overwritten by the decorator.
+        # In Python 3, we can safely return __str__ because it returns unicode.
+        return self.__str__()
+
     def is_valid(self):
         """
         checks whether the number supplied is actually valid