mime: add "image/svg+xml" type for extension ".svg"

Since there is internal usage of svg files in cmd/trace served via
HTTP, add it to the (small) map of mime types.

MIME type as documented at http://www.w3.org/TR/SVG11/mimereg.html

Change-Id: If14f07cbe2e55d8d68af663f30920a530e2e7974
Reviewed-on: https://go-review.googlesource.com/7942Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/mime/type.go

@@ -22,6 +22,7 @@ var (
 		".js":   "application/x-javascript",
 		".pdf":  "application/pdf",
 		".png":  "image/png",
+		".svg":  "image/svg+xml",
 		".xml":  "text/xml; charset=utf-8",
 	}
 	mimeTypes = clone(mimeTypesLower)