More nil interface fixes

[toffaletti]

This fixes a few more issues with encoding and decoding nil interfaces.

1. encoding nil non-empty interfaces was failing
2. encoding of nil json.Marshaler interfaces was failing
3. decoding of interfaces where the JSON value was nil was failing

Unrelated: replaced usage of should.Equal(nil, err) with should.NoError(err) in tests.

[codecov]

Codecov Report

Merging #173 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #173      +/-   ##
==========================================
+ Coverage   84.72%   84.78%   +0.05%     
==========================================
  Files          41       41              
  Lines        5199     5205       +6     
==========================================
+ Hits         4405     4413       +8     
+ Misses        633      627       -6     
- Partials      161      165       +4

Impacted Files	Coverage Δ
feature_reflect_native.go	67.7% <100%> (+2.89%)	⬆️
feature_iter_skip.go	74.69% <0%> (-3.62%)	⬇️
feature_iter_object.go	67.12% <0%> (-3.43%)	⬇️
feature_iter_string.go	88.81% <0%> (-0.7%)	⬇️
feature_reflect.go	77.97% <0%> (+0.44%)	⬆️

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