replace usage of _.extend with Object.assign

src/boxerror.js

@@ -4,8 +4,7 @@
 
 const assert = require('assert'),
     HttpError = require('connect-lastmile').HttpError,
-    util = require('util'),
-    _ = require('underscore');
+    util = require('util');
 
 exports = module.exports = BoxError;
 
@@ -28,7 +27,7 @@ function BoxError(reason, errorOrMessage, override) {
     } else { // error object
         this.message = errorOrMessage.message;
         this.nestedError = errorOrMessage;
-        _.extend(this, override); // copy enumerable properies
+        Object.assign(this, override); // copy enumerable properies
     }
 }
 util.inherits(BoxError, Error);
@@ -70,7 +69,7 @@ BoxError.TIMEOUT = 'Timeout';
 BoxError.TRY_AGAIN = 'Try Again';
 
 BoxError.prototype.toPlainObject = function () {
-    return _.extend({}, { message: this.message, reason: this.reason }, this.details);
+    return Object.assign({}, { message: this.message, reason: this.reason }, this.details);
 };
 
 // this is a class method for now in case error is not a BoxError