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

tests/integration/test_requests.py

@@ -5,7 +5,11 @@
 import os
 import pytest
 import vcr
-from assertions import assert_cassette_empty, assert_cassette_has_one_response
+from assertions import (
+    assert_cassette_empty,
+    assert_cassette_has_one_response,
+    assert_is_json
+)
 requests = pytest.importorskip("requests")
 
 
@@ -117,3 +121,19 @@ def test_cross_scheme(tmpdir, scheme):
         requests.get('http://httpbin.org/')
         assert cass.play_count == 0
         assert len(cass) == 2
+
+
+def test_gzip(tmpdir, scheme):
+    '''
+    Ensure that requests (actually urllib3) is able to automatically decompress
+    the response body
+    '''
+    url = scheme + '://httpbin.org/gzip'
+    response = requests.get(url)
+
+    with vcr.use_cassette(str(tmpdir.join('gzip.yaml'))) as cass:
+        response = requests.get(url)
+        assert_is_json(response.content)
+
+    with vcr.use_cassette(str(tmpdir.join('gzip.yaml'))) as cass:
+        assert_is_json(response.content)