add a http3.RoundTripOpt to skip the request scheme check

Otherwise, we'll only be able to issue https requests. This is what we
usually want, but for MASQUE support, the URL will be of the form
masque://example.org.

http3/roundtrip_test.go

@@ -179,6 +179,15 @@ var _ = Describe("RoundTripper", func() {
 			Expect(req.Body.(*mockBody).closed).To(BeTrue())
 		})
 
+		It("allow non-https schemes if SkipSchemeCheck is set", func() {
+			req, err := http.NewRequest("GET", "masque://www.example.org/", nil)
+			Expect(err).ToNot(HaveOccurred())
+			_, err = rt.RoundTrip(req)
+			Expect(err).To(MatchError("http3: unsupported protocol scheme: masque"))
+			_, err = rt.RoundTripOpt(req, RoundTripOpt{SkipSchemeCheck: true, OnlyCachedConn: true})
+			Expect(err).To(MatchError("http3: no cached connection was available"))
+		})
+
 		It("rejects requests without a URL", func() {
 			req1.URL = nil
 			req1.Body = &mockBody{}