Don't add coefficients in basis conversion

[blegat]

This is needed for JuliaAlgebra/MultivariateBases.jl#57.
I'm not sure how it was passing before that PR but looking at the code, it's unclear why we could need += instead of = unless we expect that there are duplicates in cfs and that it's not canonical ? I think we assume that it's always canonicalized unless inside UnsafeAddMul.
Again, let's not merge this right away, I'll check if that's the right fix

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.15%. Comparing base (dd6c877) to head (2a71232).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #85   +/-   ##
=======================================
  Coverage   84.15%   84.15%           
=======================================
  Files          15       15           
  Lines         871      871           
=======================================
  Hits          733      733           
  Misses        138      138           

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[kalmarek]

Indeed most probably I wrote this when the issue what and when is canonicalized has not yet been clarified