BUG/MINOR: server: missing default server 'resolvers' setting duplication.

'resolvers' setting was not duplicated from default server setting to
new server instances when parsing 'server' lines.
This fix is simple: strdup() default resolvers <id> string argument after
having allocated a new server when parsing 'server' lines.

This patch must be backported to 1.7 and 1.6.
diff --git a/src/server.c b/src/server.c
index a151fd4..63569cf 100644
--- a/src/server.c
+++ b/src/server.c
@@ -1813,6 +1813,8 @@
 			newsrv->agent.fall	= curproxy->defsrv.agent.fall;
 			newsrv->agent.health	= newsrv->agent.rise;	/* up, but will fall down at first failure */
 			newsrv->agent.server	= newsrv;
+			if (curproxy->defsrv.resolvers_id != NULL)
+				newsrv->resolvers_id = strdup(curproxy->defsrv.resolvers_id);
 			newsrv->dns_opts.family_prio = curproxy->defsrv.dns_opts.family_prio;
 			if (newsrv->dns_opts.family_prio == AF_UNSPEC)
 				newsrv->dns_opts.family_prio = AF_INET6;
@@ -1939,6 +1941,7 @@
 				cur_arg += 2;
 			}
 			else if (!strcmp(args[cur_arg], "resolvers")) {
+				free(newsrv->resolvers_id);
 				newsrv->resolvers_id = strdup(args[cur_arg + 1]);
 				cur_arg += 2;
 			}