fix: correct the this.isModified is not a function error

Author: mrmodise

models/user.js

@@ -23,7 +23,7 @@ const UserSchema = new Schema({
 });
 
 /** Hash the password before saving it to the database*/
-UserSchema.pre('save', (next) => {
+UserSchema.pre('save', function(next) {
     /** this refers to the user passed as argument to the save method in /routes/user*/
     const user = this;
     /** only hash the password if it has been modified or its new */
@@ -58,4 +58,4 @@ UserSchema.methods.gravatar = (size) => {
     return 'https://gravatar.com/avatar/' + md5 + '?s=' + size + '&d=retro';
 };
 /** compiling our schema into a model object - a class that constructs documents in mongoose */
-module.exports = mongoose.model('User', UserSchema);
+module.exports = mongoose.model('User', UserSchema);