Fix MatrixView inheritance

[lgoettgens]

Resolves #1685.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.54%. Comparing base (4a1abac) to head (62a0379).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1687      +/-   ##
==========================================
+ Coverage   84.53%   84.54%   +0.01%     
==========================================
  Files          94       94              
  Lines       37084    37084              
==========================================
+ Hits        31348    31353       +5     
+ Misses       5736     5731       -5     

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[thofma]

Thanks! Could you add a view(M, i, :) isa AbstractVector{...} test somewhere?