Adds failing test for nominal arff reading

The arff io reader fails to load arff files containing
a single nominal value of only 1 character.  Adds a
test demonstrating the failure in this case.
Fixes failing test of arff reader

Alteres the regex for parsing nominal values in
arff files so that single character nominal
values can now be successfully parsed.
Fixes arff io test name
Merge pull request #3073 from ghl3/arff_nominal_fix

Arff reader with nominal value of 1 character