Fix Requests so it can gunzip the request body

I wasn't emulating the stateful file-object in my response stub,
so urllib3 wasn't decompressing gzipped bodies properly.  This
should fix that problem.

Thanks @bryanhelmig for the motivation to dig into this.

tests/assertions.py

@@ -1,3 +1,6 @@
+import json
+
+
 def assert_cassette_empty(cass):
     assert len(cass) == 0
     assert cass.play_count == 0
@@ -6,3 +9,11 @@ def assert_cassette_empty(cass):
 def assert_cassette_has_one_response(cass):
     assert len(cass) == 1
     assert cass.play_count == 1
+
+
+def assert_is_json(a_string):
+    try:
+        json.loads(a_string)
+    except Exception:
+        assert False
+    assert True