Grow buffer when encoding when necessary

To maybe hopefully prevent multiple allocations

encoding/gsm7_test.go

@@ -86,6 +86,25 @@ func TestGSM7EncodeDoesNotAllocateMoreThanNecessary(t *testing.T) {
 	}
 }
 
+func TestGSM7EncodeDoesAllocateWhenNecessary(t *testing.T) {
+	coder := &GSM7Coder{}
+	input := "Ducks are fucking great, they quacks, O quackers, what the fuck."
+	var buf bytes.Buffer
+	buf.Reset()
+
+	original := buf.Cap()
+	err := coder.Encode(input, &buf)
+	modified := buf.Cap()
+
+	if err != nil {
+		t.Errorf("Expected no error, but got %v", err)
+	}
+
+	if !(modified > original) {
+		t.Errorf("Expected buffer to grow but buffer changed from %v to %v", original, modified)
+	}
+}
+
 func TestGSM7EncodeEmptyString(t *testing.T) {
 	coder := &GSM7Coder{}
 	var buf bytes.Buffer