Change specificity rules for methods involving union types

[ViralBShah]

Method signatures should look into Union types to find a more specific match.

The recent checkins in sparse.j result in a less specific version being called:

Warning: new definition +(SparseArray2d{T},Union(Array{T,N},Number)) is ambiguous with +(Tensor{S,N},Tensor{T,N}). Make sure +(SparseArray2d{S},Array{T,N}) is also defined.
Warning: new definition +(SparseArray2d{T},Union(Array{T,N},Number)) is ambiguous with +(Tensor{T,N},Number). Make sure +(SparseArray2d{T},Number) is also defined.