Add call to InitDefaults when structs implement Initializer interface during Unpack.

[blakerouse]

Call the InitDefaults during Unpack when a structure implements the Initializer interface. This allows a structure to define its default values without having to push the defaults into the loaded configuration.

Closes #104

[codecov-io]

Codecov Report

Merging #141 into master will decrease coverage by 0.34%.
The diff coverage is 82.05%.

@@            Coverage Diff            @@
##           master    #141      +/-   ##
=========================================
- Coverage   77.94%   77.6%   -0.35%     
=========================================
  Files          23      24       +1     
  Lines        2612    2643      +31     
=========================================
+ Hits         2036    2051      +15     
- Misses        425     440      +15     
- Partials      151     152       +1

Impacted Files	Coverage Δ
ucfg.go	90% <ø> (ø)	⬆️
unpack.go	85.39% <100%> (+0.33%)	⬆️
reify.go	69.83% <100%> (+1.19%)	⬆️
initializer.go	63.15% <63.15%> (ø)
merge.go	80% <0%> (-2.15%)	⬇️
path.go	83.55% <0%> (-1.98%)	⬇️
types.go	73.31% <0%> (-0.93%)	⬇️
variables.go	79.92% <0%> (-0.36%)	⬇️
util.go	92.68% <0%> (+2.43%)	⬆️

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[blakerouse]

@urso I did some more work on the PR to allow nested structures that are not defined in the configuration to still have InitDefaults to be called.

I am still having issues with getting the primitive to work properly. The InitDefaults is called on myIntInitializer that is defined in the tests, but the value is never actually set on the structure it remains the default value.

I think the issue is in tryInitDefaults:

if t.Implements(iInitializer) {
	initializer = val.Interface().(Initializer)
} else if reflect.PtrTo(t).Implements(iInitializer) {
        // reaches this branch for the myIntInitializer
	val = pointerize(reflect.PtrTo(t), t, val)
	initializer = val.Interface().(Initializer)
}
if initializer != nil {
        // calls InitDefaults
	initializer.InitDefaults()

        // val is still reflect.Zero at this point
}

I think the issue is how the field is pointerized and the value is not actually set in the address location that is defined in the structure.

[blakerouse]

@urso This is ready for another review.

[blakerouse]

@urso I have removed the need for cfgEmpty and cfgSubFlex. The behavior for pointer types in structs has been implemented.

To completely remove the need for cfgEmpty I did have to add a short circuit to unpackWith where in the case the value is nil it just returns nil and no error.