BUG/MINOR: stream: Fix how backend-specific analyzers are set on a stream

When the stream's backend was defined, the request's analyzers flag was always
set to 0 if the stream had no listener. This bug was introduced with the filter
API but never triggered (I think so).

Because of the commit 5820a366, it is now possible to encountered it. For
example, this happens when the trace filter is enabled on a SPOE backend. The
fix is pretty trivial.

This fix must be backported to 1.7.
diff --git a/src/proxy.c b/src/proxy.c
index 2d9f06c..41e40e6 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -1156,7 +1156,7 @@
 	 * be more reliable to store the list of analysers that have been run,
 	 * but what we do here is OK for now.
 	 */
-	s->req.analysers |= be->be_req_ana & (strm_li(s) ? ~strm_li(s)->analysers : 0);
+	s->req.analysers |= be->be_req_ana & ~(strm_li(s) ? strm_li(s)->analysers : 0);
 
 	/* If the target backend requires HTTP processing, we have to allocate
 	 * the HTTP transaction and hdr_idx if we did not have one.