Fix is_doc_expr by constraining number of args

[mgkurtz]

Solution as discussed in issue #538.

Without this change the first added test fails and the collect from the second test throws the BoundsError from #538 by calling iterate(::ExprSplitter).

[codecov]

Codecov Report

Merging #539 (3e23ec6) into master (7fe78fa) will decrease coverage by 1.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #539      +/-   ##
==========================================
- Coverage   86.93%   85.71%   -1.22%     
==========================================
  Files          12       12              
  Lines        2525     2415     -110     
==========================================
- Hits         2195     2070     -125     
- Misses        330      345      +15     

Impacted Files	Coverage Δ
src/utils.jl	87.55% <100.00%> (-0.73%)	⬇️
src/builtins.jl	66.24% <0.00%> (-5.53%)	⬇️
src/packagedef.jl	84.44% <0.00%> (-3.80%)	⬇️
src/types.jl	75.64% <0.00%> (-1.52%)	⬇️
src/interpret.jl	77.75% <0.00%> (-1.18%)	⬇️
src/optimize.jl	97.40% <0.00%> (-0.46%)	⬇️
src/commands.jl	93.15% <0.00%> (-0.30%)	⬇️
src/breakpoints.jl	95.55% <0.00%> (-0.24%)	⬇️
src/construct.jl	91.80% <0.00%> (-0.19%)	⬇️
... and 2 more

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[pfitzseb]

lgtm.

[timholy]

Thanks @mgkurtz !