mime: let FormatMediaType format slash-less media types, to mirror ParseMediaType.

A Content-Type always has a slash (type/subtype)
A Content-Disposition does not (e.g. "attachment" or "line").
A "media type" is either one of those, plus optional parameters afterwards.

Our ParseMediaType and FormatMediaType weren't consistent in whether
they permitted Content-Dispositions. Now they both do.

Fixes #11289

Change-Id: Ia75723c9d7adb7f4de0f65482780f823cdadb5bd
Reviewed-on: https://go-review.googlesource.com/17135Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/mime/mediatype.go

@@ -19,18 +19,21 @@ import (
 // When any of the arguments result in a standard violation then
 // FormatMediaType returns the empty string.
 func FormatMediaType(t string, param map[string]string) string {
-	slash := strings.Index(t, "/")
-	if slash == -1 {
-		return ""
-	}
-	major, sub := t[:slash], t[slash+1:]
-	if !isToken(major) || !isToken(sub) {
-		return ""
-	}
 	var b bytes.Buffer
-	b.WriteString(strings.ToLower(major))
-	b.WriteByte('/')
-	b.WriteString(strings.ToLower(sub))
+	if slash := strings.Index(t, "/"); slash == -1 {
+		if !isToken(t) {
+			return ""
+		}
+		b.WriteString(strings.ToLower(t))
+	} else {
+		major, sub := t[:slash], t[slash+1:]
+		if !isToken(major) || !isToken(sub) {
+			return ""
+		}
+		b.WriteString(strings.ToLower(major))
+		b.WriteByte('/')
+		b.WriteString(strings.ToLower(sub))
+	}
 
 	attrs := make([]string, 0, len(param))
 	for a := range param {

src/mime/mediatype_test.go

@@ -281,7 +281,7 @@ type formatTest struct {
 }
 
 var formatTests = []formatTest{
-	{"noslash", nil, ""},
+	{"noslash", map[string]string{"X": "Y"}, "noslash; x=Y"}, // e.g. Content-Disposition values (RFC 2183); issue 11289
 	{"foo bar/baz", nil, ""},
 	{"foo/bar baz", nil, ""},
 	{"foo/BAR", nil, "foo/bar"},