Missing method for == on sparse vectors

The following tries to iterate all elements of the sparse vector and takes forever:
```
julia> using SparseArrays

julia> a = SparseVector(div(typemax(Int), 2), [1], [1])
4611686018427387903-element SparseVector{Int64, Int64} with 1 stored entry:
  [1]  =  1

julia> a == a
```