net/mail: expose ParseDate, for use parsing Resent-Date headers

Fixes #16657.

Change-Id: I9425af91a48016b1d7465b9f43cafa792bc00bb3
Reviewed-on: https://go-review.googlesource.com/31581Reviewed-by: Quentin Smith <quentin@golang.org>

src/net/mail/message.go

@@ -92,7 +92,8 @@ func init() {
 	}
 }
 
-func parseDate(date string) (time.Time, error) {
+// ParseDate parses an RFC 5322 date string.
+func ParseDate(date string) (time.Time, error) {
 	for _, layout := range dateLayouts {
 		t, err := time.Parse(layout, date)
 		if err == nil {
@@ -119,7 +120,7 @@ func (h Header) Date() (time.Time, error) {
 	if hdr == "" {
 		return time.Time{}, ErrHeaderNotPresent
 	}
-	return parseDate(hdr)
+	return ParseDate(hdr)
 }
 
 // AddressList parses the named header field as a list of addresses.

src/net/mail/message_test.go

@@ -110,11 +110,16 @@ func TestDateParsing(t *testing.T) {
 		}
 		date, err := hdr.Date()
 		if err != nil {
-			t.Errorf("Failed parsing %q: %v", test.dateStr, err)
-			continue
+			t.Errorf("Header(Date: %s).Date(): %v", test.dateStr, err)
+		} else if !date.Equal(test.exp) {
+			t.Errorf("Header(Date: %s).Date() = %+v, want %+v", test.dateStr, date, test.exp)
 		}
-		if !date.Equal(test.exp) {
-			t.Errorf("Parse of %q: got %+v, want %+v", test.dateStr, date, test.exp)
+
+		date, err = ParseDate(test.dateStr)
+		if err != nil {
+			t.Errorf("ParseDate(%s): %v", test.dateStr, err)
+		} else if !date.Equal(test.exp) {
+			t.Errorf("ParseDate(%s) = %+v, want %+v", test.dateStr, date, test.exp)
 		}
 	}
 }