Fix escaping issue with @mutate

[davidanthoff]

Fixes #242.

[codecov-io]

Codecov Report

Merging #273 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #273   +/-   ##
=======================================
  Coverage   91.47%   91.47%           
=======================================
  Files           5        5           
  Lines         434      434           
=======================================
  Hits          397      397           
  Misses         37       37

Impacted Files	Coverage Δ
src/table_query_macros.jl	92.2% <100%> (ø)	⬆️

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