Fix bug in recursive proto generation where julia keywords are present.

[dewilson]

Also add a test.

[codecov]

Codecov Report

Merging #192 (8340930) into master (bae7008) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #192   +/-   ##
=======================================
  Coverage   67.95%   67.95%           
=======================================
  Files          19       19           
  Lines        2684     2684           
=======================================
  Hits         1824     1824           
  Misses        860      860           

Impacted Files	Coverage Δ
src/gen.jl	89.12% <100.00%> (ø)

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