refactor(lib/decoder): 优化变量名

[rickyes]

const headerLength = type === RpcCommandType.RESPONSE ? 20 : 22;

第一眼看去，以为是header的长度。。。。

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@f54c356). Click here to learn what that means. The diff coverage is 100%.

@@            Coverage Diff            @@
##             master       #9   +/-   ##
=========================================
  Coverage          ?   96.78%           
=========================================
  Files             ?       30           
  Lines             ?      777           
  Branches          ?        0           
=========================================
  Hits              ?      752           
  Misses            ?       25           
  Partials          ?        0

Impacted Files	Coverage Δ
lib/decoder.js	92% <100%> (ø)

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