explicitly set Content-Type in Recovery handler

writing the panic HTML content without a known content-type header
is unreliable and leads to unpredictable results - particularly when
used with other middleware that sets the header in various cases
test for non-empty body after recovered panic
Update README.md

Removed the codebot badge
Merge branch 'master' of github.com:juniorrobot/martini into juniorrobot-master