fixes before_record_response mutates response

When no cassette exists, it's expected that the response returned, should be
the original, unchanged response. The response recorded in the cassette should be
that which is returned by the before_record_response callback.

But on subsequent requests/responses (when a cassette exists), the responses
returned should be exactly what is in the cassette.

resolves #355

tests/integration/test_stubs.py

@@ -1,5 +1,6 @@
 import vcr
 import zlib
+import json
 import six.moves.http_client as httplib
 
 from assertions import assert_is_json
@@ -83,3 +84,50 @@ def test_original_decoded_response_is_not_modified(tmpdir, httpbin):
 
         assert 'content-encoding' not in inside.headers
         assert_is_json(inside.read())
+
+
+def _make_before_record_response(fields, replacement='[REDACTED]'):
+    def before_record_response(response):
+        string_body = response['body']['string'].decode('utf8')
+        body = json.loads(string_body)
+
+        for field in fields:
+            if field in body:
+                body[field] = replacement
+
+        response['body']['string'] = json.dumps(body).encode()
+        return response
+    return before_record_response
+
+
+def test_original_response_is_not_modified_by_before_filter(tmpdir, httpbin):
+    testfile = str(tmpdir.join('sensitive_data_scrubbed_response.yml'))
+    host, port = httpbin.host, httpbin.port
+    field_to_scrub = 'url'
+    replacement = '[YOU_CANT_HAVE_THE_MANGO]'
+
+    conn = httplib.HTTPConnection(host, port)
+    conn.request('GET', '/get')
+    outside = conn.getresponse()
+
+    callback = _make_before_record_response([field_to_scrub], replacement)
+    with vcr.use_cassette(testfile, before_record_response=callback):
+        conn = httplib.HTTPConnection(host, port)
+        conn.request('GET', '/get')
+        inside = conn.getresponse()
+
+        # The scrubbed field should be the same, because no cassette existed.
+        # Furthermore, the responses should be identical.
+        inside_body = json.loads(inside.read().decode('utf-8'))
+        outside_body = json.loads(outside.read().decode('utf-8'))
+        assert not inside_body[field_to_scrub] == replacement
+        assert inside_body[field_to_scrub] == outside_body[field_to_scrub]
+
+    # Ensure that when a cassette exists, the scrubbed response is returned.
+    with vcr.use_cassette(testfile, before_record_response=callback):
+        conn = httplib.HTTPConnection(host, port)
+        conn.request('GET', '/get')
+        inside = conn.getresponse()
+
+        inside_body = json.loads(inside.read().decode('utf-8'))
+        assert inside_body[field_to_scrub] == replacement