Add unit tests for Msg.Subject() method

Introduced unit tests to verify the encoding of different character sets in the Msg.Subject() method. Tests include cases for Latin characters, Japanese, Simplified Chinese, Cyrillic, and Emoji to ensure proper encoding handling.

msg_test.go

@@ -1562,6 +1562,43 @@ func TestMsg_ReplyToFormat(t *testing.T) {
 	})
 }
 
+func TestMsg_Subject(t *testing.T) {
+	tests := []struct {
+		name    string
+		subject string
+		want    string
+	}{
+		{"Normal latin characters", "Hello world!", "Hello world!"},
+		{"Empty string", "", ""},
+		{
+			"Japanese characters", `これはテスト対象です。`,
+			`=?UTF-8?q?=E3=81=93=E3=82=8C=E3=81=AF=E3=83=86=E3=82=B9=E3=83=88=E5=AF=BE?= ` +
+				`=?UTF-8?q?=E8=B1=A1=E3=81=A7=E3=81=99=E3=80=82?=`,
+		},
+		{
+			"Simplified chinese characters", `这是一个测试主题`,
+			`=?UTF-8?q?=E8=BF=99=E6=98=AF=E4=B8=80=E4=B8=AA=E6=B5=8B=E8=AF=95=E4=B8=BB?= ` +
+				`=?UTF-8?q?=E9=A2=98?=`,
+		},
+		{
+			"Cyrillic characters", `Это испытуемый`,
+			`=?UTF-8?q?=D0=AD=D1=82=D0=BE_=D0=B8=D1=81=D0=BF=D1=8B=D1=82=D1=83=D0=B5?= ` +
+				`=?UTF-8?q?=D0=BC=D1=8B=D0=B9?=`,
+		},
+		{"Emoji characters", `New Offer 🚀`, `=?UTF-8?q?New_Offer_=F0=9F=9A=80?=`},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			message := NewMsg()
+			if message == nil {
+				t.Fatal("message is nil")
+			}
+			message.Subject(tt.subject)
+			checkGenHeader(t, message, HeaderSubject, "Subject", 0, 1, tt.want)
+		})
+	}
+}
+
 // checkAddrHeader verifies the correctness of an AddrHeader in a Msg based on the provided criteria.
 // It checks whether the AddrHeader contains the correct address, name, and number of fields.
 func checkAddrHeader(t *testing.T, message *Msg, header AddrHeader, fn string, field, wantFields int,