Webmachine's wrq:peer/1 gives peername but this is replaced with
an IP address which is mentioned in X-Forwarded-For header, by WM.
This patch changes CS's behaviour, to, which does not trust the
peername unless explicitly set `true` in app.config.
add a unit test for trust_x_forwarded_for config
Merge pull request #372 from basho/bugfix/gh372-x-forwarded-for

bucket policy: use X-Forwarded-For instead of peer