Look up headers case-insensitively / allow requests with lowercase headers.

[jacksonrayhamilton]

Requests with the user-agent "Brightcove BANSHI" from zencoder.com have headers with lowercase names, which caused the error "missing host header" when parsing the lowercase "host" header from them.

HTTP headers names are case-insensitive per the spec so this is how they should be looked-up.

[ysbaddaden]

Thanks!