Pass kwargs through to HerokuCore to allow overriding session with trusted_env=False.  Workaround for Issue #33.

Use with:

    import heroku
    import requests
    session=requests.session()
    session.trust_env=False
    h=heroku.from_key('xxx', session=session)
[#46118539] switch prefetch=False to stream=True for requests lib api change
Merge pull request #32 from gabrtv/master

Heroku.from_key is silently overridden by .netrc