Add non-null to store even non-default values in serialization

[sujithvm]

Issue #, if available:

Description of changes:

• Currently non-default values are ignored in serialization based on Java defaults because of which during deserialization the defaults are reset causing different values to be used.
• For eg: if /config/audit/log_request_body is false, then it is not stored during serialization and during the deserialization, since the value is not stored, the default is reset to false.
• JsonInclude.Include.NON_NULL will store all non-null & non-default values also
• Added test

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[codecov]

Codecov Report

Merging #652 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #652      +/-   ##
============================================
+ Coverage     63.42%   63.43%   +0.01%     
- Complexity     3082     3083       +1     
============================================
  Files           234      234              
  Lines         16592    16592              
  Branches       2996     2996              
============================================
+ Hits          10523    10525       +2     
+ Misses         4529     4528       -1     
+ Partials       1540     1539       -1     

Impacted Files	Coverage Δ	Complexity Δ
...icsearch/security/auditlog/config/AuditConfig.java	98.85% <ø> (ø)	10.00 <0.00> (ø)
...icsearch/security/compliance/ComplianceConfig.java	79.86% <ø> (ø)	49.00 <0.00> (ø)
...security/configuration/DlsFlsFilterLeafReader.java	58.16% <0.00%> (-0.75%)	58.00% <0.00%> (ø%)
...transport/OpenDistroSecuritySSLRequestHandler.java	56.41% <0.00%> (+6.41%)	8.00% <0.00%> (+1.00%)

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[vrozov]

LGTM