webif: HTTP header fields must be compared case-insensitive.

According to http://tools.ietf.org/html/rfc2616#section-4.2

> Each header field consists of a name followed by a colon (":") and
> the field value. Field names are case-insensitive.

This fixes ticket #3323.

git-svn-id: http://streamboard.de.vu/svn/oscam/trunk@8654 4b0bc96b-bc66-0410-9d44-ebda105a78c1