Fix for httpbakery.Do function

When a POST request is issued, we need to replay the body contet
for the repeated request with discharged macaroons.
Changes wrt comments from rogpeppe
Fixes wrt comments from rogpeepe
Removed a comment
Merge pull request #27 from alesstimec/fix-httpbakery-replay-body-on-post

Fix for httpbakery.Do function