fix infinite loop in coroae/psgi

Perl PSGI app infinite loops since f20213ff10

Looking at that change, other loop termination conditions are '<= UWSGI_OK'.
Using gdb to inspect the app while looping shows thats ->request is returning
UWSGI_OK in this case. However, the numeric value of this is zero, so we
never exit the for(;;) loop.

Cargo culting loop escape conditions from core/async.c (or gevent/gevent.c)
fixes the inf loop for psgi/coroae.

plugins/coroae/coroae.c

@@ -161,7 +161,7 @@ request:
 #endif
 
         for(;;) {
-                if (uwsgi.p[wsgi_req->uh->modifier1]->request(wsgi_req)) {
+                if (uwsgi.p[wsgi_req->uh->modifier1]->request(wsgi_req) <= UWSGI_OK) {
                         goto end;
                 }
                 wsgi_req->switches++;