Grow buffer when encoding when necessary

To maybe hopefully prevent multiple allocations

encoding/gsm7.go

@@ -19,6 +19,11 @@ func (c *GSM7Coder) Encode(s string, buf *bytes.Buffer) error {
 		bitshift    byte = 0
 		leap, shift bool
 	)
+	encodedSize := GSM7EncodesInto(&s)
+	cap := buf.Cap()
+	if cap < encodedSize {
+		buf.Grow(encodedSize-cap)
+	}
 
 	for index, septet := range utf8 {
 		if septet > 0b01111111 {

encoding/gsm7_test.go

@@ -67,6 +67,44 @@ 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 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