minor tweak to M2M so it flattens all_columns

[dantownsend]

Resolves #728

[codecov-commenter]

Codecov Report

Merging #730 (d5e8510) into master (d5123e9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #730   +/-   ##
=======================================
  Coverage   91.22%   91.23%           
=======================================
  Files         106      107    +1     
  Lines        7292     7300    +8     
=======================================
+ Hits         6652     6660    +8     
  Misses        640      640           

Impacted Files	Coverage Δ
piccolo/columns/m2m.py	90.06% <100.00%> (+0.12%)	⬆️
piccolo/query/mixins.py	94.66% <100.00%> (-0.08%)	⬇️
piccolo/utils/list.py	100.00% <100.00%> (ø)

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