Fix bitwise error when using Extensible Filter

[nickzelei]

This change is fixing an error that occurs when the extensible filter tries to retrieve the value for dnAttributes.

This is reproducible when issuing a search with this filter:

const extFilter = new ldapjs.ExtensibleFilter({
                attribute: "userAccountControl",
                matchType: "userAccountControl",
                rule: "1.2.840.113556.1.4.803",
                value: "2",
                dnAttributes: true
});

ldapClient.search("<base dn>", { filter: extFilter }, (err, res) => { ... });

This will result in the error Expected 0x1: got 0x84

This should fix #430 and #439

[jsumners]

Are you able to add a unit test for this?

[jsumners]

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