A very small number of old browsers consider content as HTML
even when it is explicitly stated in the Content-Type header
that it is not. If content served is based on user-supplied
input, then an XSS is possible. Introduce three mitigations:

+ Don't reflect user input in error strings
+ Set a Content-Disposition header when requesting a resource
  that should never be displayed in a browser window
+ Set X-Content-Type-Options: nosniff on all responses

Change-Id: I81c9d6736e0439ebd1db99cd7fb701cc56d24805
Reviewed-on: https://go-review.googlesource.com/102318
Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>