Allow DefaultArrayStyle to be broadcast according to normal rules

[jdeldre]

Hot fix to #137

The original issue lies in the way broadcasting works for zeros(), since zeros().+2.0 gets turned from Array{Float64, 0} into Float64, because apparently that is how broadcasting is set up for DefaultArrayStyle{0} (This seems like a bug in Julia.) Before #136, this wasn't a problem for broadcasting of ArrayPartition as long as an Array{Float64, 0} was paired with another component of higher Array rank, e.g., ap = ArrayPartition(zeros(),[1.0]), since they would all be broadcast via DefaultArrayStyle{1} rules. So ap+1.0 would be of the same type as ap. But on its own, e.g., ap = ArrayPartition(zeros()), then ap + 1.0 would not be the same type as ap, since it gets broadcast as a DefaultArrayStyle{0}. Unfortunately, #136 exposed this issue.

This PR provides a more narrowly defined scope for when components of ArrayPartition will change their style in unpack from that of the promoted style of the overall Broadcasted wrapper of the ArrayPartition. If it is of DefaultArrayStyle, then it keeps the overall wrapper's style, thereby preserving the behavior before #136. But if the ArrayPartition contains a custom type with its own BroadcastStyle, then it parses the components' styles and therefore still allows specialized dispatch on copy.