Fix contentstream package golint notices

[adrg]

Fix golint notices in the contentstream package.
Addresses #421

[codecov]

Codecov Report

Merging #422 into v3 will increase coverage by <.01%.
The diff coverage is 73.33%.

@@            Coverage Diff             @@
##               v3     #422      +/-   ##
==========================================
+ Coverage   54.37%   54.38%   +<.01%     
==========================================
  Files         153      153              
  Lines       27680    27677       -3     
==========================================
+ Hits        15050    15051       +1     
+ Misses      10834    10830       -4     
  Partials     1796     1796

Impacted Files	Coverage Δ
pdf/contentstream/processor.go	50.3% <ø> (ø)	⬆️
pdf/contentstream/inline-image.go	33.97% <50%> (-0.13%)	⬇️
pdf/contentstream/parser.go	65.53% <79.16%> (+0.74%)	⬆️

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