add some extra assertions to ensure all mappers are gone after clear_mappers() (for [ticket:963])
add some extra assertions to ensure all mappers are gone after clear_mappers() (for [ticket:963])

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