Slicing a BlockIndexRange with unit ranges doesn't return a BlockedIndexRange

[mtfishman]

For example:

julia> Block(1, 1)[1:4, 1:4]
4×4 BlockArrays.BlockIndexRange{2, Tuple{UnitRange{Int64}, UnitRange{Int64}}}:
 Block(1, 1)[1, 1]  Block(1, 1)[1, 2]  Block(1, 1)[1, 3]  Block(1, 1)[1, 4]
 Block(1, 1)[2, 1]  Block(1, 1)[2, 2]  Block(1, 1)[2, 3]  Block(1, 1)[2, 4]
 Block(1, 1)[3, 1]  Block(1, 1)[3, 2]  Block(1, 1)[3, 3]  Block(1, 1)[3, 4]
 Block(1, 1)[4, 1]  Block(1, 1)[4, 2]  Block(1, 1)[4, 3]  Block(1, 1)[4, 4]

julia> (Block(1, 1)[1:4, 1:4])[2:3, 2:3]
2×2 Matrix{BlockIndex{2}}:
 Block(1, 1)[2, 2]  Block(1, 1)[2, 3]
 Block(1, 1)[3, 2]  Block(1, 1)[3, 3]

while it could return Block(1, 1)[2:3, 2:3]. This would be helpful for implementing #346, among other applications.