`HeadInject` lets you put stuff into the HTML head of every page in
the wrapped application::
>>> from testrunner import send_request
>>> from headinject import HeadInject
>>> from simpleapp import simple_app
>>> wrapped_app = HeadInject(
... simple_app, '')
>>> print send_request(wrapped_app, '/')
200 OK
Content-Type: text/html
Simple App
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