memrecycle fails to coerce complex to character

[sritchie73]

Current behaviour:

A = data.table(A=as.complex(1))
B = data.table(A="foo")
rbind(A,B)
# Error in rbindlist(l, use.names, fill, idcol) : 
#   Unsupported type 'complex' passed to allNA()

Expected:

rbind(A,B)
#         A
#    <char>
# 1:   0+0i
# 2:    foo