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/113396Reviewed-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>

src/net/http/sniff.go

@@ -147,6 +147,8 @@ var sniffSignatures = []sniffSig{
 	&exactSig{[]byte("\x50\x4B\x03\x04"), "application/zip"},
 	&exactSig{[]byte("\x1F\x8B\x08"), "application/x-gzip"},
 
+	&exactSig{[]byte("\x00\x61\x73\x6D"), "application/wasm"},
+
 	mp4Sig{},
 
 	textSig{}, // should be last

src/net/http/sniff_test.go

@@ -65,6 +65,7 @@ var sniffTests = []struct {
 
 	{"woff sample  I", []byte("\x77\x4f\x46\x46\x00\x01\x00\x00\x00\x00\x30\x54\x00\x0d\x00\x00"), "font/woff"},
 	{"woff2 sample", []byte("\x77\x4f\x46\x32\x00\x01\x00\x00\x00"), "font/woff2"},
+	{"wasm sample", []byte("\x00\x61\x73\x6d\x01\x00"), "application/wasm"},
 }
 
 func TestDetectContentType(t *testing.T) {