Make a case-insensitive comparision for the pattern "chunked" for "Transfer-Encoding"

While verifying the value of the "Transfer-Encoding" header, libhtp does a
case-sensitive comparison for the value "chunked", thus pushing out
case-sensitive values such as "Chunked".

Convert the case-sensitive comparison to a case-insensitive one by using
bstr_cmp_c_nocase(), instead of the currently used bstr_cmp_c().