Sub filter: fixed matching after a partial match.

Sub filter: fixed matching after a partial match.

After a failed partial match we now check if there is another partial
match in previously matched substring to fix cases like "aab" in "aaab".

The ctx->saved string is now always sent if it's present on return
from the ngx_http_sub_parse() function (and reset accordingly).  This
allows to release parts of previously matched data.