Fix parsing of legacy url_rewrite_program responses (#2420)

Fix parsing of legacy url_rewrite_program responses (#2420)

Legacy helper responses start with a URL instead of `OK rewrite_url=...`
and such. 2016 commit ddc77a2e introduced two bugs when handling legacy
responses:

* Response parsing code triggered MemBuf assertions when 0-terminating
  the parsing buffer for certain URLs. The bug affected legacy helper
  responses with and without space characters.

* Squid code attempted to accept/use helper-returned URLs with embedded
  space character(s), despite a WARNING implying that the post-space
  characters are not going to become a part of the new URL.