BUG/MINOR: http-fetch: http_auth_bearer() must not match a missing header

BUG/MINOR: http-fetch: http_auth_bearer() must not match a missing header

When http_auth_bearer() was called with a custom header name and this header
was not found, the fetch still succeeded and returned an empty string. So a
condition like "{ http_auth_bearer(x-tok) -m found }" was true for a request
without any such header.

Commit 909578520 ("BUG/MINOR: http-fetch: Fix http_auth_bearer() when custom
header is used") intended to report a mismatch in this case, as its message
says, but only did it for a header with a wrong type.

Let's return 0 when the header is not found. A reg-test is added for the
fetch with and without a custom header.

This should be backported as far as 2.6.