Correctly deal when PaginatedList's data is a dict (#2084)

When calculating totalCount for a PaginatedList, if a Link header is not
in the returned data, we use len() to calculate the number of items.
PullRequest.get_review_requests() actually returns a dictionary, which
neatly defeats this naive method. Peer inside the dictionary in this
case, and add a test case.

Fixes #2053

github/PaginatedList.py

@@ -153,6 +153,8 @@ class PaginatedList(PaginatedListBase):
                 if data and "total_count" in data:
                     self.__totalCount = data["total_count"]
                 elif data:
+                    if isinstance(data, dict):
+                        data = data[self.__list_item]
                     self.__totalCount = len(data)
                 else:
                     self.__totalCount = 0