add more tests for encoding using the static table

[marten-seemann]

As suggested in #10 (comment).

[codecov-commenter]

Codecov Report

Merging #15 into master will increase coverage by 0.13%.
The diff coverage is 92.59%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   91.13%   91.27%   +0.13%     
==========================================
  Files           4        5       +1     
  Lines         203      229      +26     
==========================================
+ Hits          185      209      +24     
- Misses          9       11       +2     
  Partials        9        9              

Impacted Files	Coverage Δ
static_table.go	0.00% <0.00%> (ø)
encoder.go	100.00% <100.00%> (ø)

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