Fix how we access needs_hashing prop in password field, fixes #4020

fields/types/password/PasswordType.js

@@ -68,23 +68,23 @@ password.prototype.addToSchema = function (schema) {
 	schema.path(this.path, _.defaults({
 		type: String,
 		set: function (newValue) {
-			this.set(needs_hashing, true);
+			this[needs_hashing] = true;
 			return newValue;
 		},
 	}, this.options));
 
 	schema.virtual(this.paths.hash).set(function (newValue) {
 		this.set(field.path, newValue);
-		this.set(needs_hashing, false);
+		this[needs_hashing] = false;
 	});
 
 	schema.pre('save', function (next) {
-		if (!this.isModified(field.path) || !this.get(needs_hashing)) {
+		if (!this.isModified(field.path) || !this[needs_hashing]) {
 			return next();
 		}
 		if (!this.get(field.path)) {
 			this.set(field.path, undefined);
-			this.set(needs_hashing, false);
+			this[needs_hashing] = false;
 			return next();
 		}
 		var item = this;
@@ -100,7 +100,7 @@ password.prototype.addToSchema = function (schema) {
 				item.set(field.path, hash);
 				// reset [needs_hashing] so that new values can't be hashed more than once
 				// (inherited models double up on pre save handlers for password fields)
-				item.set(needs_hashing, false);
+				item[needs_hashing] = false;
 				next();
 			});
 		});