Sanchez/include prompt flag

@gradyward @wedaly 

Simple change to include a flag that drops the prompt from rubric serialization, for the new editing panel. Tests updated accordingly.

Incidentally, ended up changing the encoding flag on all our serialization from "utf-8" to "unicode" to change the return value from a byte string to a unicode. 
@gradyward this is the added line to verify the prompt is not in the serialized string.