Fix promotion of outer product

[blegat]

This is important when using MultivariatePolynomials.Term to infer that the eltype of the result is Term and not Polynomial.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2a7b7ca) 89.52% compared to head (35358ab) 90.95%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #263      +/-   ##
==========================================
+ Coverage   89.52%   90.95%   +1.43%     
==========================================
  Files          22       23       +1     
  Lines        2148     2167      +19     
==========================================
+ Hits         1923     1971      +48     
+ Misses        225      196      -29     

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