Add additional unit tests

[sudo-jaa]

I've been taking a look at your project and have identified some classes which could potentially benefit from increased code coverage. The regression tests contained in this PR should catch some edge cases that haven't yet been covered by your existing tests for these classes. They'll help you identify any unexpected behaviour in your code as you continue to develop your project.

I'd be happy to help create some more tests for your project should you wish!

[codecov-io]

Codecov Report

Merging #217 into master will increase coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #217      +/-   ##
==========================================
+ Coverage   68.31%   68.46%   +0.14%     
==========================================
  Files         107      107              
  Lines        7334     7334              
  Branches     1388     1388              
==========================================
+ Hits         5010     5021      +11     
+ Misses       1868     1867       -1     
+ Partials      456      446      -10

Impacted Files	Coverage Δ
src/main/java/com/jsoniter/spi/OmitValue.java	96.51% <0%> (+9.3%)	⬆️
src/main/java/com/jsoniter/spi/Slice.java	96.42% <0%> (+10.71%)	⬆️

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