improvement(broker): don't reject unrestricted client if there are no restricted proxies

I.e. match it with an unrestricted proxy (if there is one).

The old behavior exists since the inception of the restricted vs
unrestricted feature, i.e. 0052c0e10c

broker/snowflake-broker_test.go

@@ -167,6 +167,30 @@ func TestBroker(t *testing.T) {
 				So(w.Code, ShouldEqual, http.StatusOK)
 			})
 
+			Convey("with unrestricted proxy to unrestricted client if there are no restricted proxies", func() {
+				snowflake := ctx.AddSnowflake("test", "", NATUnrestricted, 0)
+				offerData, err := createClientOffer(sdp, NATUnrestricted, "")
+				So(err, ShouldBeNil)
+				r, err := http.NewRequest("POST", "snowflake.broker/client", offerData)
+
+				done := make(chan bool)
+				go func() {
+					clientOffers(i, w, r)
+					done <- true
+				}()
+
+				select {
+				case <-snowflake.offerChannel:
+				case <-time.After(250 * time.Millisecond):
+					So(false, ShouldBeTrue)
+					return
+				}
+				snowflake.answerChannel <- "test answer"
+
+				<-done
+				So(w.Body.String(), ShouldEqual, `{"answer":"test answer"}`)
+			})
+
 			Convey("Times out when no proxy responds.", func() {
 				if testing.Short() {
 					return