fix: excluded_* and required_* checks aren't triggered on pointers to structs

[iradmt]

Fixes Or Enhances

Fix for #906
Trigger excluded_* and required_* checks on pointers to structs.
Before fix all default backed-in tags were ignored for non-nil pointers to structs.
Now it's possible to check whether pointer to struct is nil or not depending on another field value.

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• Tests exist or have been written that cover this particular change.

@go-playground/validator-maintainers

[coveralls]

coverage: 73.975% (+0.003%) from 73.972% when pulling 3453fec on iradmt:bugfix/nil_checkable_on_struct_ptr into bd1113d on go-playground:master.