net/http: empty contenty-type treated as application/octet-stream

RFC 2616, section 7.2.1 - empty type SHOULD be treated as
application/octet-stream.
Fixes #6616.

R=golang-codereviews, gobot, bradfitz, josharian
CC=golang-codereviews
https://golang.org/cl/31810043

src/pkg/net/http/request.go

@@ -673,6 +673,11 @@ func parsePostForm(r *Request) (vs url.Values, err error) {
 		return
 	}
 	ct := r.Header.Get("Content-Type")
+	// RFC 2616, section 7.2.1 - empty type
+	//   SHOULD be treated as application/octet-stream
+	if ct == "" {
+		ct = "application/octet-stream"
+	}
 	ct, _, err = mime.ParseMediaType(ct)
 	switch {
 	case ct == "application/x-www-form-urlencoded":

src/pkg/net/http/request_test.go

@@ -68,8 +68,9 @@ type parseContentTypeTest struct {
 
 var parseContentTypeTests = []parseContentTypeTest{
 	{false, stringMap{"Content-Type": {"text/plain"}}},
-	// Non-existent keys are not placed. The value nil is illegal.
-	{true, stringMap{}},
+	// Empty content type is legal - shoult be treated as
+	// application/octet-stream (RFC 2616, section 7.2.1)
+	{false, stringMap{}},
 	{true, stringMap{"Content-Type": {"text/plain; boundary="}}},
 	{false, stringMap{"Content-Type": {"application/unknown"}}},
 }