(THRIFT-61) Fix "make check" for the C++ library by not doing empty flushes.

(THRIFT-61) Fix "make check" for the C++ library by not doing empty flushes.

I changed the behavior of TFramedTransport when flush is called without
writing any data to the transport.  This broke the unit test, which was
relying on a weird corner of TFramedTransport's behavior in order to do
some stricter checking.  I altered the unit test to never flush without
writing and added a new test to verfy the "empty flush" behavior.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@672881 13f79535-47bb-0310-9956-ffa450edef68