msgpack: add more testcases

[zchee]

Add more testcases to msgpack and msgpack/rpc packages.

[codecov]

Codecov Report

Merging #91 (7556bc0) into master (726b60e) will increase coverage by 2.2%.
The diff coverage is 60.2%.

@@           Coverage Diff            @@
##           master     #91     +/-   ##
========================================
+ Coverage    60.6%   62.8%   +2.2%     
========================================
  Files          12      13      +1     
  Lines        2079    2080      +1     
========================================
+ Hits         1260    1307     +47     
+ Misses        684     636     -48     
- Partials      135     137      +2     

Flag	Coverage Δ
linux-1.13	62.8% <60.2%> (+2.2%)	⬆️
linux-1.14	62.8% <60.2%> (+2.2%)	⬆️
linux-1.15	62.8% <60.2%> (+2.2%)	⬆️
linux-1.16	62.8% <60.2%> (+2.2%)	⬆️
macos-1.13	62.8% <60.2%> (+2.2%)	⬆️
macos-1.14	62.8% <60.2%> (+2.2%)	⬆️
macos-1.15	62.8% <60.2%> (+2.2%)	⬆️
macos-1.16	62.8% <60.2%> (+2.2%)	⬆️

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Impacted Files	Coverage Δ
msgpack/field.go	71.0% <0.0%> (ø)
msgpack/unpack.go	90.6% <0.0%> (+4.6%)	⬆️
msgpack/decode.go	75.9% <50.0%> (+11.5%)	⬆️
msgpack/rpc/rpc.go	58.0% <57.8%> (ø)
msgpack/msgpack.go	66.6% <66.6%> (ø)
msgpack/pack.go	87.5% <66.6%> (ø)
msgpack/encode.go	76.7% <100.0%> (ø)

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