Make false check explicit in isBerWriter comparison

Co-authored-by: James Sumners <[email protected]>

Author: rkaw92

lib/filters/filter.js

@@ -45,7 +45,7 @@ function isBerWriter (ber) {
 
 function mixin (target) {
   target.prototype.toBer = function toBer (ber) {
-    if (!isBerWriter(ber)) { throw new TypeError('ber (BerWriter) required') }
+    if (isBerWriter(ber) === false) { throw new TypeError('ber (BerWriter) required') }
 
     ber.startSequence(TYPES[this.type])
     ber = this._toBer(ber)