Add memory allocation test

To make sure none is happening

encoding/gsm7_test.go

@@ -67,6 +67,25 @@ func TestGSM7EncodeComplex8nASCIIString(t *testing.T) {
 	}
 }
 
+func TestGSM7EncodeDoesNotAllocateMoreThanNecessary(t *testing.T) {
+	coder := &GSM7Coder{}
+	input := "Ducks are fucking great, they quacks, O quackers, what the fuck."
+	buf := bytes.NewBuffer(make([]byte, GSM7EncodesInto(&input)))
+	buf.Reset()
+
+	expected := buf.Cap()
+	err := coder.Encode(input, buf)
+	actual := buf.Cap()
+
+	if err != nil {
+		t.Errorf("Expected no error, but got %v", err)
+	}
+
+	if actual != expected {
+		t.Errorf("Expected buffer of size %v, but got %v", expected, actual)
+	}
+}
+
 func TestGSM7EncodeEmptyString(t *testing.T) {
 	coder := &GSM7Coder{}
 	var buf bytes.Buffer