Allow registry clients to connect via http2

Http2 will be enabled by default and can be disabled with a configuration option.

Signed-off-by: Adam Duke <adam.v.duke@gmail.com>

registry/registry_test.go

@@ -0,0 +1,30 @@
+package registry
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/docker/distribution/configuration"
+)
+
+// Tests to ensure nextProtos returns the correct protocols when:
+// * config.HTTP.HTTP2.Disabled is not explicitly set => [h2 http/1.1]
+// * config.HTTP.HTTP2.Disabled is explicitly set to false [h2 http/1.1]
+// * config.HTTP.HTTP2.Disabled is explicitly set to true [http/1.1]
+func TestNextProtos(t *testing.T) {
+	config := &configuration.Configuration{}
+	protos := nextProtos(config)
+	if !reflect.DeepEqual(protos, []string{"h2", "http/1.1"}) {
+		t.Fatalf("expected protos to equal [h2 http/1.1], got %s", protos)
+	}
+	config.HTTP.HTTP2.Disabled = false
+	protos = nextProtos(config)
+	if !reflect.DeepEqual(protos, []string{"h2", "http/1.1"}) {
+		t.Fatalf("expected protos to equal [h2 http/1.1], got %s", protos)
+	}
+	config.HTTP.HTTP2.Disabled = true
+	protos = nextProtos(config)
+	if !reflect.DeepEqual(protos, []string{"http/1.1"}) {
+		t.Fatalf("expected protos to equal [http/1.1], got %s", protos)
+	}
+}