Fix error when logging http response with python 3

Python 3 changed the semantics of dict.items() [0], which now returns a
iterable 'view' instead of a list of tuples. This has the repercussion
that you can no longer check for membership of a key using:

    key in dict.items()

This patch simply replaces that check with a test for the key existing
in the dict itself, rather than the items.

[0] http://legacy.python.org/dev/peps/pep-3106/

Closes-Bug: 1359880

Change-Id: I7c59b0432725b660c9fa7270cde2e07bf3ea77db