fix #43
Was a bug in the method for sdcMicroObj... is fixed now

This works now:
data(testdata)
sdc <- createSdcObj(testdata,
    keyVars=c('urbrur','roof','walls','water','electcon','relat','sex'), 
    numVars=c('expend','income','savings'), w='sampling_weight')
sdc <- mafast(sdc,by=c("urbrur","roof"))
sdc <- mafast(sdc,by=c("urbrur"))


sdc <- createSdcObj(testdata,
    keyVars=c('urbrur','roof','electcon','relat','sex'), 
    numVars=c('expend','income','savings'), w='sampling_weight')
sdc <- mafast(sdc,by=c("urbrur","roof","walls"))
sdc <- mafast(sdc,by=c("urbrur","water"))
sdc <- mafast(sdc,by=c("walls","water"))