eWAY Payment Processor fails when a naked ampersand is input

If the request sent to eWAY by the eWAY Payment Processor contains a naked '&' then eWAY responds with ...

Payment Processor Error message
: 9008:Error: XML not formated correctly to eWAY standards. Reason:Whitespace is not allowed at this location. Error Code:-1072896749

The problem occurs because the request sent to eWAY is in XML format, and eWAY expects the '&' to be represented as '&'

I've attached a patch to packages/eWAY/eWAY_GatewayRequest.php which fixes the problem.

Gotchas:
1. I notice that the character '<' is already converted to '>' by the time eWAY_GatewayRequest.php acts on it, so the final string sent to eWAY is '&gt;'. Not sure why '&' causes a problem but '<' is handled OK. Perhaps that points to a better solution.
2. I think this is a safe change, as the 'hard' fields (CC number, CVN, Expiry Month, and Expiry Year) are all numbers and shouldn't contain ampersands.