mime: change *.js mime type to application/javascript, not x-javascript

We delayed doing this for 4 years for fear that it might break something,
but it was standardized (RFC 4329) 12 years ago, and the default in Debian
and other places is correct:

   $ cat /etc/mime.types  | grep js$
   application/javascript                          js

Time for us to change too.

I doubt there will be problems, but we'll see during the Go 1.11 beta.

Fixes #7498

Change-Id: Iba0bf8a6e707a64dd63317e1c0d6dd9a18634527
Reviewed-on: https://go-review.googlesource.com/120058Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/mime/type.go

@@ -62,7 +62,7 @@ var builtinTypesLower = map[string]string{
 	".htm":  "text/html; charset=utf-8",
 	".html": "text/html; charset=utf-8",
 	".jpg":  "image/jpeg",
-	".js":   "application/x-javascript",
+	".js":   "application/javascript",
 	".wasm": "application/wasm",
 	".pdf":  "application/pdf",
 	".png":  "image/png",