Adds support for optional request body Fixes gh-126

[matt62king]

trafficstars

Adds support for @RequestBody(required = false)

When request body is optional and null, encoder will add an empty body by default.

Fixes gh-126

[codecov[bot]]

Codecov Report

Merging #263 into master will decrease coverage by 1.93%. The diff coverage is 85.71%.

@@             Coverage Diff              @@
##             master     #263      +/-   ##
============================================
- Coverage     79.09%   77.15%   -1.94%     
- Complexity      351      373      +22     
============================================
  Files            42       51       +9     
  Lines          1320     1497     +177     
  Branches        202      219      +17     
============================================
+ Hits           1044     1155     +111     
- Misses          190      246      +56     
- Partials         86       96      +10     

Impacted Files	Coverage Δ	Complexity Δ
...amework/cloud/openfeign/support/SpringEncoder.java	78.33% <50.00%> (-11.95%)	12.00 <0.00> (-6.00)
...eign/annotation/RequestBodyParameterProcessor.java	90.90% <90.90%> (ø)	5.00 <5.00> (?)
...ork/cloud/openfeign/support/SpringMvcContract.java	82.18% <100.00%> (-6.65%)	47.00 <0.00> (-9.00)
...ringframework/cloud/openfeign/DefaultTargeter.java	0.00% <0.00%> (-100.00%)	0.00% <0.00%> (-2.00%)
...ork/cloud/openfeign/support/PageJacksonModule.java	37.50% <0.00%> (-16.67%)	4.00% <0.00%> (ø%)
...mework/cloud/openfeign/FeignAutoConfiguration.java	88.88% <0.00%> (-3.42%)	3.00% <0.00%> (ø%)
...mework/cloud/openfeign/FeignClientFactoryBean.java	67.64% <0.00%> (-1.51%)	46.00% <0.00%> (-3.00%)
...amework/cloud/openfeign/FeignClientProperties.java	57.97% <0.00%> (-0.94%)	7.00% <0.00%> (ø%)
... and 22 more

[OlgaMaciaszek]

Have created another issue in OpenFeign: https://github.com/OpenFeign/feign/issues/1383.