codec: do not dereference pointers/interfaces for omitempty support.

[notnoop]

This backports empty value checking changes from https://github.com/ugorji/go that are found in 006e153 and 66dd47f .

Without this change, omitempty treats pointer to empty values as empty (e.g. if a field is *bool and value is a reference to false), and treating a struct pointer is empty if all elements of struct as empty (e.g. if a field is of type *struct { A string }, then nil and &{""} values would be treated as empty and omitted).

This fixes the issue to be inline with json/encoding - where pointer is treated as empty and omitted only if it's a nil pointer.

Original Comment description from 66dd47f :

This is in line with the standard set by encoding/json
i.e. if omitempty=true, pointers/interfaces are omitted iff nil.

Previously, I "mistakenly" thought that
encoding/json dereferenced pointers/interfaces when checking if empty.

Fixes ugorji#67

I've also added a test for the case that wasn't in original repo. The style of repo tests is a bit odd, and I conformed with it rather than establish a new test style.

[mkeeler]

LGTM

For others who may look and are wondering what this changes I have dug in and figured it out.

Without this change *bool pointer values that were non-nil and pointed to a false value were being omitted from the encoding. With this change it allows it to only omit the value when the pointer is nil and if not will always encode the bool value.

[notnoop]

Thanks! Sorry I should have elaborated in the description a bit more -- Without this change, omitempty used to treat pointer to empty values as empty (e.g. if a field is *bool and value is a reference to false), and treating a struct pointer is empty if all elements of struct as empty (e.g. if a field is of type *struct { A string }, then nil and &{""} values would be treated as empty and omitted).

This fixes the issue to be inline with json/encoding - where pointer is treated as empty and omitted only if it's a nil pointer.