Make all BigDecimal :to_xml use the to_xml_big_decimal helper method.

Making all the `BigDecimal` `:to_xml` calls use the helper method so that we don't have a bunch of Procs strewn about the cdebase as well as making the overall codebase more consistent.

Thoughts about extending `BigDecimal` to just add a `:to_xml` method so we don't have to even have the code: `, :to_xml => to_xml_big_decimal`? Not sure if you averse to polluting base classes or not.

### Test restuls
```bash
|2.0.0-p247| Marks-MacBook-Pro in ~/quickbooks-ruby
± |bigdecimal ✗| → rake spec

Finished in 1.13 seconds
187 examples, 0 failures
Coverage report generated for RSpec to /Users/mrickert/quickbooks-ruby/coverage. 1730 / 1809 LOC (95.63%) covered.
```