Added xmlrpc request string as object property

[enrico-sorcinelli]

Hi,
this PR aims to add XML-RPC request string as object property _xmlrcp_request_as_string after the simple_request() method call.
In fact, if first argument of the method above isn't a RPC::XML::request object itself (so we can use as_string method), it seems there isn't a possibility to retrieve the string sent to the server.

[rjray]

Merged. I noted that you misspelled the property as _xmlrcp_request_as_string rather than _xmlrpc_request_as_string. I'll fix that.

[rjray]

This has me looking at this code again after several years. I realize that your patch could lead to some memory issues if a person is (for example) streaming a really large file in a request. I'm going to add to your code so that this is controlled by a configuration parameter in RPC::XML::Client::new(). I hope that this will still meet your needs.