Fix blocks in popfirst

[jishnub]

Fixes the following behaviour on master:

julia> B = BlockArray([1:6;], [2,2,2])
3-blocked 6-element BlockVector{Int64}:
 1
 2
 ─
 3
 4
 ─
 5
 6

julia> popfirst!(B)
1

julia> B
3-blocked 6-element BlockVector{Int64}:
               2
 ───────────────
               3
               4
 139721335802992
 ───────────────
               5
               6

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.06%. Comparing base (ea33af1) to head (29404a3).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #344      +/-   ##
==========================================
+ Coverage   92.66%   93.06%   +0.40%     
==========================================
  Files          16       16              
  Lines        1499     1499              
==========================================
+ Hits         1389     1395       +6     
+ Misses        110      104       -6     

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