Adjust $_SERVER[CONTENT_TYPE] to ignore if the server is not setting that

Line 520 broke an existing implementation I had running because the $_SERVER['CONTENT_TYPE'] is not being set by my server.  I'm not sure if this is a wide ranging issue, but this fixed the problem for me locally.
Merge pull request #46 from lanji/patch-1

In `env` function, check if $_SERVER[CONTENT_TYPE] exists before testing its value