Fix error detecting content type

[jacegu]

This is a consequence of #94. That PR didn't consider the charset that may be part of the Content-Type header.

[weppos]

Makes sense! 👍

FYI, if we need a simple match at the beginning of a string, I'd use String.start_with? which is slightly more efficient than a regexp match.

[jacegu]

if we need a simple match at the beginning of a string, I'd use String.start_with? which is slightly more efficient than a regexp match.

👍 Changing it. Is there somewhere where I can read about this (in other words: how do you know it's more efficient)?

[weppos]