fix: use '=' in args

Author: CatWithApple

lib/argument_parser.js

@@ -740,7 +740,7 @@ ArgumentParser.prototype._parseOptional = function (argString) {
   if (argString.indexOf('=') >= 0) {
     var argStringSplit = argString.split('=');
     optionString = argStringSplit[0];
-    argExplicit = argStringSplit[1];
+    argExplicit = argStringSplit.slice(1).join('=');
 
     if (!!this._optionStringActions[optionString]) {
       action = this._optionStringActions[optionString];