net/textproto: skip zero-length keys

A header of ": value" results in an empty key. Do not add
it to the headers, because RFC7230 (section 3.2) says that
field-names are tokens, which are one or more characters.

Fixes #11205.

Change-Id: I883be89da1489dc84f98523786b019d1d0169d46
Reviewed-on: https://go-review.googlesource.com/11242Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/textproto/reader.go

@@ -485,6 +485,13 @@ func (r *Reader) ReadMIMEHeader() (MIMEHeader, error) {
 		}
 		key := canonicalMIMEHeaderKey(kv[:endKey])
 
+		// As per RFC 7230 field-name is a token, tokens consist of one or more chars.
+		// We could return a ProtocolError here, but better to be liberal in what we
+		// accept, so if we get an empty key, skip it.
+		if key == "" {
+			continue
+		}
+
 		// Skip initial spaces in value.
 		i++ // skip colon
 		for i < len(kv) && (kv[i] == ' ' || kv[i] == '\t') {

src/net/textproto/reader_test.go

@@ -153,6 +153,15 @@ func TestReadMIMEHeaderSingle(t *testing.T) {
 	}
 }
 
+func TestReadMIMEHeaderNoKey(t *testing.T) {
+	r := reader(": bar\ntest-1: 1\n\n")
+	m, err := r.ReadMIMEHeader()
+	want := MIMEHeader{"Test-1": {"1"}}
+	if !reflect.DeepEqual(m, want) || err != nil {
+		t.Fatalf("ReadMIMEHeader: %v, %v; want %v", m, err, want)
+	}
+}
+
 func TestLargeReadMIMEHeader(t *testing.T) {
 	data := make([]byte, 16*1024)
 	for i := 0; i < len(data); i++ {