Bug fix and tests improvements

[mtakagi]

• Fix Unkeyed encode bug.
• Separate tests file.
• And more tests.

[codecov-io]

Codecov Report

Merging #11 into develop will increase coverage by 7.64%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop      #11      +/-   ##
===========================================
+ Coverage    77.41%   85.06%   +7.64%     
===========================================
  Files            9       10       +1     
  Lines         1116     1165      +49     
  Branches        62       62              
===========================================
+ Hits           864      991     +127     
+ Misses         248      170      -78     
  Partials         4        4

Impacted Files	Coverage Δ
Tests/MsgPackEncoderTests/SingleValueTests.swift	100% <100%> (ø)
...ests/MsgPackEncoderTests/MsgPackEncoderTests.swift	100% <100%> (ø)	⬆️
Sources/MsgPackEncoder/MsgPackEncoder.swift	78.66% <100%> (+12.92%)	⬆️

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