using Long.valueOf instead of casting double to long

[dyf6372]

When parsing some big number like 6643122506645376263, the result(6643122506645376000) will not be satisfying.

[codecov-io]

Codecov Report

Merging #219 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
+ Coverage   68.31%   68.31%   +<.01%     
==========================================
  Files         107      107              
  Lines        7334     7335       +1     
  Branches     1388     1388              
==========================================
+ Hits         5010     5011       +1     
  Misses       1868     1868              
  Partials      456      456

Impacted Files	Coverage Δ
src/main/java/com/jsoniter/JsonIterator.java	65.01% <100%> (+0.13%)	⬆️

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