Bug: Block indexing changed between 0.16.40 and 0.16.43

Hi all, 

I believe a bug was introduced somewhere 0.16.40 and 0.16.43. 

See the following MWE: 

```julia 
using BlockArrays

x = mortar([[1,2,3],[4,5,6]])
@assert blocks(x)[1] === x[Block(1)]
```

This was true in 0.16.40, and breaks for 0.16.43.
I believe `x[Block(1)]` is allocating a new array now. I assume it's a bug.

Thanks, 
Jordi