Fix GetLastError to match the spec.
Fix GetLastError to match the spec.

http://docs.mongodb.org/manual/reference/command/getLastError/#dbcmd.getLastError
maxime-gautre/ReactiveMongo,mcveat/ReactiveMongo,lunatech-labs/lunatech-reactivemongo,cchantep/ReactiveMongo,maowug/ReactiveMongo,ReactiveMongo/ReactiveMongo,cchantep/ReactiveMongo,maxime-gautre/ReactiveMongo,avdv/ReactiveMongo,sh1ng/ReactiveMongo,reactific/ReactiveMongo,avdv/ReactiveMongo,charleskubicek/ReactiveMongo,charleskubicek/ReactiveMongo,mcveat/ReactiveMongo,ornicar/ReactiveMongo,viktortnk/ReactiveMongo,bfil/ReactiveMongo,ReactiveMongo/ReactiveMongo,maowug/ReactiveMongo,viktortnk/ReactiveMongo,ornicar/ReactiveMongo