Fix missing six.text_type() call on APIException.__str__ (#5476)

Pull up method from subclass to superclass

The call was added in 426547c
to allow for dict-style arguments to ValidationError but does not
apply to other APIException descendants.

Author: jleclanche

rest_framework/exceptions.py

@@ -92,7 +92,7 @@ def __init__(self, detail=None, code=None):
         self.detail = _get_error_details(detail, code)
 
     def __str__(self):
-        return self.detail
+        return six.text_type(self.detail)
 
     def get_codes(self):
         """
@@ -136,9 +136,6 @@ def __init__(self, detail=None, code=None):
 
         self.detail = _get_error_details(detail, code)
 
-    def __str__(self):
-        return six.text_type(self.detail)
-
 
 class ParseError(APIException):
     status_code = status.HTTP_400_BAD_REQUEST