Change to RequestSizeLimiter & better tests

[gabstv]

Fixes #2
This PR also makes the tests self contained (removed external server dependency);

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@6d03d1b). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master      #4   +/-   ##
========================================
  Coverage          ?   87.5%           
========================================
  Files             ?       1           
  Lines             ?      40           
  Branches          ?       0           
========================================
  Hits              ?      35           
  Misses            ?       3           
  Partials          ?       2

Impacted Files	Coverage Δ
size.go	87.5% <100%> (ø)

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