Allow unions to contain fields with names of "primitive" types

 	Fix the ability for unions to contain fields with names of "primitive" types, e.g. Byte, Short, Int, Long, etc.

  *  The problem here was that when generating the union, the ScalaGenerator would create case classes with the names of the union's fields.  This would cause the declaration of a variable to use tthe local case classes instead of the scala.* versions. T
  *  This fix makes a trade off of a little more code in ths fix, for the benefit of having the generated code look better.  Mainly, instead of fully-qualifying all declarations, we only do it in a few spots.
  *  Repro case is added to naughty.thrift