Fixed invalid usage of Module#undef_method
Fixed invalid usage of Module#undef_method

* In MRI Module#undef_method accepts 0 or more method names, and removes
  them all. In JRuby it accepts only 1 method name. The documentation
  shows that only 1 method name should be provided, so JRuby matches the
  docs, while MRI has a much looser contract.

  DM was using the MRI API, but this has been changed to match the
  documented behaviour, and should work in JRuby.

[#1229 state:resolved]

tillsc/dm-core,tpitale/dm-core,datamapper/dm-core,alloy/dm-core,ar-dm/ardm-core,engineyard/dm-core