[BUG] Mixed scalar and block indexing is broken

These slicing operations are broken:
```julia
using BlockArrays: Block
using BlockSparseArrays: BlockSparseArray
a = BlockSparseArray{Float64}([2, 2], [2, 2])
a[1, Block(1)]
a[1, Block.(1:2)]
```
however the following appear to work:
```julia
a[1:1, Block(1)]
a[1:1, Block.(1:2)]
```