[core] better consistency in buffer_is_equal*()
[core] better consistency in buffer_is_equal*()

buffer_is_equal_caseless_string() now correctly matches against
the string only up to the provided string length, since the
string might not be '\0' terminated.

gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4