FIX: Workaround for integer hashing
FIX: Workaround for integer hashing

Force conversion to integer for missing values when
they must be integer to avoid hash errors on 32 bit
platforms.

closes #8968

toobaz/pandas,linebp/pandas,rs2/pandas,winklerand/pandas,datapythonista/pandas,jmmease/pandas,linebp/pandas,zfrenchee/pandas,louispotok/pandas,amolkahat/pandas,gfyoung/pandas,datapythonista/pandas,jmmease/pandas,toobaz/pandas,jmmease/pandas,toobaz/pandas,cbertinato/pandas,cython-testbed/pandas,DGrady/pandas,harisbal/pandas,MJuddBooth/pandas,winklerand/pandas,jmmease/pandas,GuessWhoSamFoo/pandas,jorisvandenbossche/pandas,cbertinato/pandas,TomAugspurger/pandas,cython-testbed/pandas,zfrenchee/pandas,louispotok/pandas,louispotok/pandas,kdebrab/pandas,jmmease/pandas,MJuddBooth/pandas,toobaz/pandas,pratapvardhan/pandas,gfyoung/pandas,louispotok/pandas,amolkahat/pandas,DGrady/pandas,zfrenchee/pandas,winklerand/pandas,pratapvardhan/pandas,MJuddBooth/pandas,nmartensen/pandas,gfyoung/pandas,MJuddBooth/pandas,dsm054/pandas,jorisvandenbossche/pandas,nmartensen/pandas,Winand/pandas,GuessWhoSamFoo/pandas,GuessWhoSamFoo/pandas,DGrady/pandas,jreback/pandas,kdebrab/pandas,DGrady/pandas,harisbal/pandas,linebp/pandas,DGrady/pandas,nmartensen/pandas,GuessWhoSamFoo/pandas,cbertinato/pandas,louispotok/pandas,cython-testbed/pandas,nmartensen/pandas,linebp/pandas,Winand/pandas,winklerand/pandas,nmartensen/pandas,zfrenchee/pandas,cbertinato/pandas,pandas-dev/pandas,pratapvardhan/pandas,amolkahat/pandas,TomAugspurger/pandas,pratapvardhan/pandas,amolkahat/pandas,kdebrab/pandas,DGrady/pandas,jorisvandenbossche/pandas,TomAugspurger/pandas,Winand/pandas,harisbal/pandas,winklerand/pandas,jreback/pandas,jreback/pandas,rs2/pandas,dsm054/pandas,GuessWhoSamFoo/pandas,toobaz/pandas,nmartensen/pandas,datapythonista/pandas,dsm054/pandas,pandas-dev/pandas,cbertinato/pandas,kdebrab/pandas,harisbal/pandas,zfrenchee/pandas,rs2/pandas,dsm054/pandas,cython-testbed/pandas,amolkahat/pandas,linebp/pandas,rs2/pandas,gfyoung/pandas,jorisvandenbossche/pandas,kdebrab/pandas,Winand/pandas,Winand/pandas,jreback/pandas,jmmease/pandas,jreback/pandas,TomAugspurger/pandas,MJuddBooth/pandas,pandas-dev/pandas,winklerand/pandas,cython-testbed/pandas,datapythonista/pandas,dsm054/pandas,pandas-dev/pandas,harisbal/pandas,linebp/pandas,gfyoung/pandas,Winand/pandas,pratapvardhan/pandas