Fix banded-diagonal Lmul/Rmul copy

[jishnub]

This works around the missing convert methods to Bidiagonal/Tridiagonal, and fixes the failing tests on nightly

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.53%. Comparing base (0690818) to head (abdb8b9).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/diagonal.jl	87.50%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #255      +/-   ##
==========================================
- Coverage   89.58%   89.53%   -0.05%     
==========================================
  Files          11       11              
  Lines        1930     1940      +10     
==========================================
+ Hits         1729     1737       +8     
- Misses        201      203       +2     

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[jishnub]

Missing coverage seems spurious, as the added lines are definitely being tested. Commenting them out causes the tests to fail.