Remove instanceof check in filter#toBer mixin

For #629

lib/filters/filter.js

@@ -2,14 +2,10 @@
 
 // var assert = require('assert')
 
-var asn1 = require('asn1')
-
 var Protocol = require('../protocol')
 
 /// --- Globals
 
-var BerWriter = asn1.BerWriter
-
 var TYPES = {
   and: Protocol.FILTER_AND,
   or: Protocol.FILTER_OR,
@@ -40,7 +36,7 @@ function isFilter (filter) {
 
 function mixin (target) {
   target.prototype.toBer = function toBer (ber) {
-    if (!ber || !(ber instanceof BerWriter)) { throw new TypeError('ber (BerWriter) required') }
+    if (!ber) { throw new TypeError('ber (BerWriter) required') }
 
     ber.startSequence(TYPES[this.type])
     ber = this._toBer(ber)