Re-enable HTTP/2 for Go 1.7.

 * Since Go 1.7 HTTP/2 support is enabled only if TLSConfig.NextProtos
   includes the string "h2".
 * see mholt/caddy#975

caddyhttp/httpserver/server.go

@@ -92,6 +92,10 @@ func NewServer(addr string, group []*SiteConfig) (*Server, error) {
 	if err != nil {
 		return nil, err
 	}
+	// Since Go 1.7 HTTP/2 is enabled only if TLSConfig.NextProtos includes the string "h2".
+	if HTTP2 && s.Server.TLSConfig != nil && len(s.Server.TLSConfig.NextProtos) == 0 {
+		s.Server.TLSConfig.NextProtos = []string{"h2"}
+	}
 
 	// Compile custom middleware for every site (enables virtual hosting)
 	for _, site := range group {