net/http: add application/wasm mime type

net/http: add application/wasm mime type

Although not part of http://mimesniff.spec.whatwg.org,
for WASM streaming compilation to happen, the response
needs to have the application/wasm MIME type
as mentioned here:
https://webassembly.github.io/spec/web-api/index.html#streaming-modules.

And all current browsers prevent streaming compilation
from happening if this MIME type is not present in the response.

The magic number is mentioned here:
https://webassembly.org/docs/binary-encoding

Since we are already adding WASM support, it makes sense
to support this MIME type.

Change-Id: I8dd7b413a8c438a5c23c29d843b42f6da2a20ba4
Reviewed-on: https://go-review.googlesource.com/113396
Reviewed-by: Richard Musiol <neelance@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>