fix double nan error when parse to json

[ylwu-amzn]

Issue #, if available:

Description of changes:
new Gson().toJson(Double.NaN) will throw such exception: "NaN is not a valid double value as per JSON specification"
This PR fixed this bug

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[codecov]

Codecov Report

Merging #310 (e2da1a2) into master (75db58b) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #310      +/-   ##
============================================
- Coverage     72.23%   72.20%   -0.04%     
+ Complexity     1993     1992       -1     
============================================
  Files           201      201              
  Lines          9567     9567              
  Branches        849      849              
============================================
- Hits           6911     6908       -3     
- Misses         2245     2247       +2     
- Partials        411      412       +1     

Flag	Coverage Δ	Complexity Δ
plugin	71.63% <100.00%> (-0.04%)	0.00 <1.00> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
...stroforelasticsearch/ad/AnomalyDetectorPlugin.java	94.82% <100.00%> (ø)	11.00 <1.00> (ø)
...port/SearchAnomalyDetectorInfoTransportAction.java	54.54% <0.00%> (-4.55%)	4.00% <0.00%> (ø%)
...asticsearch/ad/cluster/ADClusterEventListener.java	92.00% <0.00%> (-2.00%)	14.00% <0.00%> (-1.00%)