allow caller to provide the ioloop

UserAgent.pm.diff

--- /usr/share/perl5/vendor_perl/Mojo/UserAgent.pm      2017-03-13 17:13:52.000000000 +0000
+++ /home/kgoess/mylib/lib/perl5/Mojo/UserAgent.pm      2017-07-12 22:46:18.000000000 +0000
@@ -43,21 +43,21 @@
 
 sub start {
   my ($self, $tx, $cb) = @_;
 
   # Fork-safety
   $self->_cleanup->server->restart unless ($self->{pid} //= $$) eq $$;
 
   # Non-blocking
   if ($cb) {
     warn "-- Non-blocking request (@{[_url($tx)]})\n" if DEBUG;
-    return $self->_start(Mojo::IOLoop->singleton, $tx, $cb);
+    return $self->_start($self->ioloop, $tx, $cb);
   }
 
   # Blocking
   warn "-- Blocking request (@{[_url($tx)]})\n" if DEBUG;
   $self->_start($self->ioloop, $tx => sub { shift->ioloop->stop; $tx = shift });
   $self->ioloop->start;
 
   return $tx;
 }