Allow empty body to be valid for jrpc.HandleCall/Deserialize; allow method override in client

[ldemailly]

Allow empty body to be valid for HandleCall/Deserialize; allow call/send/... methods to be overridden

[codecov]

Codecov Report

Base: 88.4% // Head: 88.4% // Decreases project coverage by -0.1% ⚠️

Coverage data is based on head (dfe8ce9) compared to base (0d61b4f).
Patch coverage: 100.0% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #623     +/-   ##
========================================
- Coverage    88.4%   88.4%   -0.1%     
========================================
  Files          30      30             
  Lines        4313    4320      +7     
========================================
+ Hits         3814    3818      +4     
- Misses        335     337      +2     
- Partials      164     165      +1     

Impacted Files	Coverage Δ
jrpc/jrpcServer.go	100.0% <ø> (ø)
jrpc/jrpcClient.go	100.0% <100.0%> (ø)
dflag/configmap/updater.go	78.8% <0.0%> (-2.9%)	⬇️

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