Preserve axes in similar for BlockArrays

[jishnub]

Fixes #420

After this,

julia> using BlockArrays

julia> using BlockArrays: BlockedOneTo

julia> b = BlockVector([1,2,3,4,5,6,7,8,9,10], (BlockedOneTo(5:5:10),));

julia> typeof(b) == typeof(zero(b))
true

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.47%. Comparing base (836da5c) to head (1ce8185).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #422      +/-   ##
==========================================
- Coverage   96.66%   96.47%   -0.19%     
==========================================
  Files          18       18              
  Lines        1649     1647       -2     
==========================================
- Hits         1594     1589       -5     
- Misses         55       58       +3     

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