Fix MA.add_mul for GenericNonlinearExpr arguments

[odow]

This should let us merge jump-dev/MutableArithmetics.jl#225 without breakage.

The default fallback in MA is annoying for types that have different representations of a + b and b + a:
jump-dev/MutableArithmetics.jl#225 (comment)

[codecov]

Codecov Report

Patch coverage: 85.71% and project coverage change: -0.09% ⚠️

Comparison is base (37d5b70) 98.09% compared to head (0f42f5c) 98.00%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3488      +/-   ##
==========================================
- Coverage   98.09%   98.00%   -0.09%     
==========================================
  Files          37       37              
  Lines        5520     5525       +5     
==========================================
  Hits         5415     5415              
- Misses        105      110       +5     

Files Changed	Coverage Δ
src/nlp_expr.jl	99.04% <75.00%> (-0.24%)	⬇️
src/aff_expr.jl	96.96% <100.00%> (-0.36%)	⬇️
src/mutable_arithmetics.jl	92.17% <100.00%> (+0.28%)	⬆️

... and 9 files with indirect coverage changes

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