Fix for https://github.com/aws/aws-cli/issues/302.

Fix for https://github.com/aws/aws-cli/issues/302.

Even though we have an explicit dependency on requests version 1.2.0, some users are running into trouble with version 1.2.3.  This commit in requests:

https://github.com/kennethreitz/requests/commit/f7596c75dce4e87ab83bdf74e8f120a4b1a5ff03

seems to have changed the behavior wrt the ``response.headers``.  Previously, if you requested a header name which didn't exist in the response headers, it would return a None.  Now, you get a ``KeyError``.  So, this PR changes the reference from a dict lookup to a ``get`` call.