godoc/util: serve SVG files raw

When a godoc Presentation is serving a file, it will sniff its
contents to determine if it contains "human-readable" text. If
it does, then it serves the contents of the file with surrounding
HTML. SVG files should not be served with any additional HTML even
though their contents are not a binary format (just like CSS or JS
files).

Updates golang/go#9936

Change-Id: Id3dd3b6cda4f103b08f6e924923426a86bd46ec8
Reviewed-on: https://go-review.googlesource.com/c/tools/+/177498
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

godoc/util/util.go

@@ -59,6 +59,7 @@ func IsText(s []byte) bool {
 var textExt = map[string]bool{
 	".css": false, // must be served raw
 	".js":  false, // must be served raw
+	".svg": false, // must be served raw
 }
 
 // IsTextFile reports whether the file has a known extension indicating