Fix similar for DefaultDict

Right now, calling similar on a DefaultDict doesn't work:

    julia> a = DefaultDict(0)
    DefaultDict{Any,Any,Int64}()

    julia> similar(a)
    ERROR: no method DefaultDict{Any,Any,Int64}()

The reason for this problem is that you have to specify a default value when constructing a DefaultDict. This patch uses the same default value of the original DefaultDict for the new one.