net/http: remove Content-Encoding header in roundtrip_js

The fetch api will decode the gzip, but Content-Encoding not be deleted.
To ensure that the behavior of roundtrip_js is consistent with native. delete the Content-Encoding header when the response body is decompressed by js fetch api.

Fixes golang#63139

Change-Id: Ie35b3aa050786e2ef865f9ffa992e30ab060506e
Reviewed-on: https://go-review.googlesource.com/c/go/+/530155
Commit-Queue: Damien Neil <dneil@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>

Author: 0x5457

src/net/http/roundtrip_js.go

@@ -10,6 +10,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"net/http/internal/ascii"
 	"strconv"
 	"strings"
 	"syscall/js"
@@ -184,11 +185,22 @@ func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 		}
 
 		code := result.Get("status").Int()
+
+		uncompressed := false
+		if ascii.EqualFold(header.Get("Content-Encoding"), "gzip") {
+			// The fetch api will decode the gzip, but Content-Encoding not be deleted.
+			header.Del("Content-Encoding")
+			header.Del("Content-Length")
+			contentLength = -1
+			uncompressed = true
+		}
+
 		respCh <- &Response{
 			Status:        fmt.Sprintf("%d %s", code, StatusText(code)),
 			StatusCode:    code,
 			Header:        header,
 			ContentLength: contentLength,
+			Uncompressed:  uncompressed,
 			Body:          body,
 			Request:       req,
 		}