Fix assertions in constructor

[bkamins]

@nalimilan I propose to revert b3036f8 you have made. I do not understand why it is needed and currently it leads to an incorrect result:

julia> PooledArray([1 2; 3 4], Int8)
ERROR: TypeError: in typeassert, expected Vector{Int8}, got a value of type Matrix{Int8}

[bkamins]

OK - I see you wanted the constructor to be type stable

[codecov]

Codecov Report

Merging #80 (5026aef) into main (53f57d4) will increase coverage by 1.51%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
+ Coverage   87.00%   88.51%   +1.51%     
==========================================
  Files           1        1              
  Lines         331      331              
==========================================
+ Hits          288      293       +5     
+ Misses         43       38       -5     

Impacted Files	Coverage Δ
src/PooledArrays.jl	88.51% <100.00%> (+1.51%)	⬆️

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[bkamins]

Thank you!