Looser accept-encoding in header check.

Fixes #628

Apparently some sites don't like the `requests` library's default Accept-Encoding header ("gzip, deflate, compress") for HEAD requests. Setting it to '*' works.

(Today I learned how to enable logging for the requests library -- see http://stackoverflow.com/questions/10588644/how-can-i-see-the-entire-request-thats-being-sent-to-paypal-in-my-python-applic )