added a unit test for nested session transactions
added a unit test for nested session transactions

brianv0/sqlalchemy,bootandy/sqlalchemy,halfcrazy/sqlalchemy,wujuguang/sqlalchemy,inspirehep/sqlalchemy,Cito/sqlalchemy,epa/sqlalchemy,monetate/sqlalchemy,wfxiang08/sqlalchemy,276361270/sqlalchemy,alex/sqlalchemy,elelianghh/sqlalchemy,monetate/sqlalchemy,zzzeek/sqlalchemy,sandan/sqlalchemy,brianv0/sqlalchemy,alex/sqlalchemy,j5int/sqlalchemy,davidfraser/sqlalchemy,bdupharm/sqlalchemy,itkovian/sqlalchemy,WinterNis/sqlalchemy,Akrog/sqlalchemy,EvaSDK/sqlalchemy,epa/sqlalchemy,dstufft/sqlalchemy,davidjb/sqlalchemy,alex/sqlalchemy,hsum/sqlalchemy,robin900/sqlalchemy,bdupharm/sqlalchemy,EvaSDK/sqlalchemy,ThiefMaster/sqlalchemy,elelianghh/sqlalchemy,wfxiang08/sqlalchemy,pdufour/sqlalchemy,davidjb/sqlalchemy,hsum/sqlalchemy,itkovian/sqlalchemy,graingert/sqlalchemy,olemis/sqlalchemy,Cito/sqlalchemy,sqlalchemy/sqlalchemy,halfcrazy/sqlalchemy,Akrog/sqlalchemy,Cito/sqlalchemy,ThiefMaster/sqlalchemy,sandan/sqlalchemy,robin900/sqlalchemy,bootandy/sqlalchemy,inspirehep/sqlalchemy,276361270/sqlalchemy,davidfraser/sqlalchemy,pdufour/sqlalchemy,j5int/sqlalchemy,WinterNis/sqlalchemy,dstufft/sqlalchemy,graingert/sqlalchemy,wujuguang/sqlalchemy,olemis/sqlalchemy