BUG/MINOR: http-fetch: fix the space check of http_auth_bearer(<hdr>)

BUG/MINOR: http-fetch: fix the space check of http_auth_bearer(<hdr>)

When http_auth_bearer() was called with a custom header name, a test was
performed to know if the value contained a space after the scheme, by
comparing the length of the scheme with the length of the header value.

But istsplit() had already advanced <ctx.value> past the scheme and the
space at this point, so the comparison was made with the length of the token
instead:

  - a token of exactly 6 characters, the length of "Bearer", was reported as
    a mismatch, e.g. "Bearer abcdef";

  - a lone "Bearer" without any space was accepted and returned an empty
    token.

Let's compare with the length of the value before it is split. The reg-test
is extended with these two cases.

This should be backported to all supported versions.