bug Client: use content-type (charset) to decode response body
bug Client: use content-type (charset) to decode response body

We've been assuming all along that the response body was encoded as
ASCII. While that is questionable all by itself, upgrading to Python 3.0
will force us to do explicit decoding; so we may as well get it right
now.

Introduces a new method Client.get_charset to parse the content-type
header and return the charset

jpaugh/agithub,mozilla/agithub