Fix support of get_full_details() for Throttled exceptions (#4627)

Since `str` objects are immutable, appending to existing `str` creates
in fact a new `str` instance.

Thus `ErrorDetail.detail.code` attribute is lost after `str` concatenation operation.

rest_framework/exceptions.py

@@ -210,14 +210,14 @@ class Throttled(APIException):
     default_code = 'throttled'
 
     def __init__(self, wait=None, detail=None, code=None):
+        if detail is None:
+            detail = force_text(self.default_detail)
+        if wait is not None:
+            wait = math.ceil(wait)
+            detail = ' '.join((
+                detail,
+                force_text(ungettext(self.extra_detail_singular.format(wait=wait),
+                                     self.extra_detail_plural.format(wait=wait),
+                                     wait))))
+        self.wait = wait
         super(Throttled, self).__init__(detail, code)
-
-        if wait is None:
-            self.wait = None
-        else:
-            self.wait = math.ceil(wait)
-            self.detail += ' ' + force_text(ungettext(
-                self.extra_detail_singular.format(wait=self.wait),
-                self.extra_detail_plural.format(wait=self.wait),
-                self.wait
-            ))

tests/test_exceptions.py

 from __future__ import unicode_literals
 
 from django.test import TestCase
+from django.utils import six
 from django.utils.translation import ugettext_lazy as _
 
-from rest_framework.exceptions import ErrorDetail, _get_error_details
+from rest_framework.exceptions import (
+    ErrorDetail, Throttled, _get_error_details
+)
 
 
 class ExceptionTestCase(TestCase):
@@ -39,3 +42,18 @@ class ExceptionTestCase(TestCase):
             _get_error_details([[lazy_example]])[0][0],
             ErrorDetail
         )
+
+    def test_get_full_details_with_throttling(self):
+        exception = Throttled()
+        assert exception.get_full_details() == {
+            'message': 'Request was throttled.', 'code': 'throttled'}
+
+        exception = Throttled(wait=2)
+        assert exception.get_full_details() == {
+            'message': 'Request was throttled. Expected available in {} seconds.'.format(2 if six.PY3 else 2.),
+            'code': 'throttled'}
+
+        exception = Throttled(wait=2, detail='Slow down!')
+        assert exception.get_full_details() == {
+            'message': 'Slow down! Expected available in {} seconds.'.format(2 if six.PY3 else 2.),
+            'code': 'throttled'}