fix coeff on divisions

[mxhbl]

This is a partial fix to issue #925, making coeff usable on divisions if the target symbol is not in the denominator.
This PR will result in coeff(x / y, x) returning 1 / y, instead of throwing an error, while coeff(x / (y + x), x) will still throw a not implemented error.

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Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.56%. Comparing base (79c4e92) to head (62859ce).
Report is 320 commits behind head on master.

Files	Patch %	Lines
src/utils.jl	75.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1224      +/-   ##
==========================================
- Coverage   77.07%   76.56%   -0.52%     
==========================================
  Files          32       36       +4     
  Lines        3533     3746     +213     
==========================================
+ Hits         2723     2868     +145     
- Misses        810      878      +68     

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[ChrisRackauckas]

oh awesome! Yeah that looks good.