Revision 719727

THRIFT-142. java: Better handling of required fields

- On reading, behave the same way as the C++ code:
  throw an exception if a required field is missing.
- In addition, throw an exception if a required field is missing
  when writing.  For the JavaBeans code, this means that __isset
  is false (because it is maintained automatically).  For non-beans
  code, this means that the field is null.  Non-nullable fields are
  not checked in non-beans code.