Make client request base64 encoding

This makes gotty-client still work.

webtty/codecs.go

@@ -0,0 +1,19 @@
+package webtty
+
+type Decoder interface {
+	Decode(dst, src []byte) (int, error)
+}
+
+type Encoder interface {
+	Encode(dst, src []byte) (int, error)
+}
+
+type NullCodec struct{}
+
+func (NullCodec) Encode(dst, src []byte) (int, error) {
+	return copy(dst, src), nil
+}
+
+func (NullCodec) Decode(dst, src []byte) (int, error) {
+	return copy(dst, src), nil
+}