CLEANUP: http_ana: Make use of the return value of stream_generate_unique_id()

CLEANUP: http_ana: Make use of the return value of stream_generate_unique_id()

Even if `unique_id` and `s->unique_id` are identical it is a bit odd to
`isttest()` `unique_id` and then use `s->unique_id` in the call to `http_add_header()`.

This "issue" was introduced in a17e66289c08a5bfadc1bb5b5f2c618c9299fe1b,
because before that commit the function returned the length of the ID, as it
was not an ist.