Implement sniffing for fonts:
* MS Font object  --> "application/vnd.ms-fontobject"
* ttf		  --> "application/font-ttf"

* off		  --> "application/font-off"
* otf		  --> "application/font-off"

* cff		  --> "application/font-cff"
* woff		  --> "application/font-woff"

Fixes #20808

Change-Id: Ibe02a87d3c9d610c6a30e1b6c03f4e520404e70f
Reviewed-on: https://go-review.googlesource.com/47553
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>