Make valueMap to be static to prevent multiple warning

[jiachen1120]

related issue: #627

[codecov-io]

Codecov Report

Merging #628 into 1.6.x will decrease coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##              1.6.x     #628      +/-   ##
============================================
- Coverage     50.73%   50.71%   -0.02%     
+ Complexity     1912     1910       -2     
============================================
  Files           258      258              
  Lines         11492    11492              
  Branches       1622     1622              
============================================
- Hits           5830     5828       -2     
  Misses         4975     4975              
- Partials        687      689       +2

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/com/networknt/config/ConfigInjection.java	74.39% <100%> (ø)	25 <0> (ø)	⬇️
...n/java/com/networknt/client/oauth/OauthHelper.java	36.44% <0%> (-0.61%)	26% <0%> (-2%)

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[stevehu]

I have tested with the petstore example and it works perfectly. Thanks