fix checking a surrogate pair

[t-mangoe]

I fixed code. I think this fix solves the issue #207 . If you like, please merge.

[codecov-io]

Codecov Report

Merging #232 into master will decrease coverage by <.01%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #232      +/-   ##
==========================================
- Coverage   68.46%   68.45%   -0.01%     
==========================================
  Files         107      107              
  Lines        7335     7336       +1     
  Branches     1388     1388              
==========================================
  Hits         5022     5022              
- Misses       1867     1868       +1     
  Partials      446      446

Impacted Files	Coverage Δ
...ain/java/com/jsoniter/output/StreamImplString.java	53.91% <0%> (-0.48%)	:arrow_down:

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[plokhotnyuk]

@t-mangoe please add tests for that case

best option would be just checking of a whole range of surrogate pairs in some loop with an assertion for each pair