Code cleanup and add more test cases.

[ujjboy]

Modification:

1. Fix some possible NPE.
2. Add more test case.

[codecov-io]

Codecov Report

Merging #177 into master will increase coverage by 1.93%.
The diff coverage is 64%.

@@             Coverage Diff              @@
##             master     #177      +/-   ##
============================================
+ Coverage        69%   70.94%   +1.93%     
+ Complexity      955      954       -1     
============================================
  Files           341      341              
  Lines         14194    14196       +2     
  Branches       2278     2281       +3     
============================================
+ Hits           9795    10071     +276     
+ Misses         3235     2947     -288     
- Partials       1164     1178      +14

Impacted Files	Coverage Δ	Complexity Δ
.../java/com/alipay/sofa/rpc/ext/ExtensionLoader.java	81.77% <ø> (-0.1%)	0 <0> (ø)
.../sofa/rpc/client/aft/impl/TimeWindowRegulator.java	81.91% <ø> (ø)	7 <0> (ø)	⬇️
.../java/com/alipay/sofa/rpc/server/BusinessPool.java	85.71% <ø> (+58.44%)	0 <0> (ø)	⬇️
...lipay/sofa/rpc/config/AbstractInterfaceConfig.java	46.39% <0%> (-0.43%)	0 <0> (ø)
.../java/com/alipay/sofa/rpc/config/MethodConfig.java	53.65% <0%> (-2.76%)	0 <0> (ø)
...va/com/alipay/sofa/rpc/server/bolt/BoltServer.java	72.22% <0%> (-1.65%)	23 <0> (ø)
...lipay/sofa/rpc/message/AbstractResponseFuture.java	53.57% <100%> (ø)	0 <0> (ø)	⬇️
...va/com/alipay/sofa/rpc/server/rest/RestServer.java	80.76% <100%> (+17.04%)	0 <0> (ø)	⬇️
...om/alipay/sofa/rpc/common/json/BeanSerializer.java	66.27% <100%> (ø)	0 <0> (ø)	⬇️
...in/java/com/alipay/sofa/rpc/common/SystemInfo.java	92.85% <100%> (+29.22%)	0 <0> (ø)	⬇️
... and 26 more

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