Add HTTPCompressorWithLevel

Also use the new flate package for gzip compression for HTTP,
and set the default HTTP compression level to 4, which is
currently the best efficiency in V2.

Author: andybalholm

http.go

@@ -1,17 +1,22 @@
 package brotli
 
 import (
-	"compress/gzip"
 	"io"
 	"net/http"
 	"strings"
+
+	"github.com/andybalholm/brotli/flate"
 )
 
 // HTTPCompressor chooses a compression method (brotli, gzip, or none) based on
 // the Accept-Encoding header, sets the Content-Encoding header, and returns a
 // WriteCloser that implements that compression. The Close method must be called
 // before the current HTTP handler returns.
 func HTTPCompressor(w http.ResponseWriter, r *http.Request) io.WriteCloser {
+	return HTTPCompressorWithLevel(w, r, 4)
+}
+
+func HTTPCompressorWithLevel(w http.ResponseWriter, r *http.Request, level int) io.WriteCloser {
 	if w.Header().Get("Vary") == "" {
 		w.Header().Set("Vary", "Accept-Encoding")
 	}
@@ -20,10 +25,10 @@ func HTTPCompressor(w http.ResponseWriter, r *http.Request) io.WriteCloser {
 	switch encoding {
 	case "br":
 		w.Header().Set("Content-Encoding", "br")
-		return NewWriterV2(w, DefaultCompression)
+		return NewWriterV2(w, level)
 	case "gzip":
 		w.Header().Set("Content-Encoding", "gzip")
-		return gzip.NewWriter(w)
+		return flate.NewGZIPWriter(w, level)
 	}
 	return nopCloser{w}
 }