Using DosingInstructions.getDosingInstructionsAsString when creating a DrugOrder display. Include a display with DrugOrder representations.

This is to include dosing instructions in the standard display of a drug order.  Discussed with @djazayeri for a use case in a pharmacy system being built at github.com/openmrs/openmrs-module-ebolaexample.

I looked at other Resource commits to base my unit tests, let me know if there's a different approach I should've taken.  Also, I left in a lot of the null checks that were already in the display method, didn't fully investigate if they were still necessary.
Don't actually need the Representation.REF here, since this is a plain (String) object, not an OpenMRS one that needs to be converted.