THRIFT-1394:Treatment of optional fields is not consistent between C++ and Java

THRIFT-1394:Treatment of optional fields is not consistent between C++ and Java
Client: cpp
Patch: Diwaker Gupta

In CPP, all optional fields are guarded by the isset helper struct. On Java, however, the generated code takes advantage of nullable types: for containers, structs, exceptions, enums, and, notably, strings, the generator elides explicit use of an "isset" bit vector and instead emits checks of the form "field null". This leads to varying behavior between the two languages: an optional string field with a default value will have {{isset[fieldid]}} false on C, but the equivalent test in Java will be true.

git-svn-id: https://svn.apache.org/repos/asf/thrift/trunk@1236529 13f79535-47bb-0310-9956-ffa450edef68