Instantiating ListingMapper on 1.9.2 results in Stack Level Too Deep

# How to Reproduce

Assuming an empty gemset

```
$> gem install rets4r
$> irb
ruby-1.9.2 > require 'rets4r'
ruby-1.9.2 > RETS4R::ListingMapper.new
```

Produces:

```
SystemStackError: stack level too deep
    from /Users/jschairb/.rvm/rubies/ruby-1.9.2-p180/lib/ruby/1.9.1/irb/workspace.rb:80
Maybe IRB bug!!
```
