Constant enum not treated as constant in java encoder

[kdkavanagh]

Generated encoder put method for constant enum field incorrectly mutates the underlying buffer at offset 0. Decoder works as expected.

  <message id="11" name="NewOrderSingleMarket" description="New Market Order inbound to Aquarius">
    <field description="" id="1" name="OrdType" presence="constant" type="OrdType" valueRef="OrdType.MarketWithProtection"/>

  public NewOrderSingleMarketEncoder ordType(OrdType value) {
    this.buffer.putByte(this.offset + 0, (byte)value.value());
    return this;
  }