Overwrite and default logic does not work with bool pointer

[capri-xiyue]

{
			name: "bool_pointer_overwrite_default",
			input: &struct {
				Field *bool `env:"FIELD,noinit,overwrite,default=true"`
			}{
				Field: new(bool),
			},
			exp: &struct {
				Field *bool `env:"FIELD,noinit,overwrite,default=true"`
			}{
				Field: new(bool),
			},
			lookuper: MapLookuper(nil),
		},

I wrote test cases as above and it failed.
It does not follow the logic listed in https://github.com/sethvargo/go-envconfig#overwrite
If the struct field has a non-zero value and a default is set and if no environment variable is specified, the struct field's existing value will be used (the default is ignored).

[sethvargo]

Fixed in #73