don't mark path as dirty when using custom getter

Just a quick update to prevent mongoose to mark a path as dirty when using a custom getter 

right now when you do something like that mongoose mark the path as modified 
this PR fix that by using  `getValue`  instead of `get` to compare the new value with the priorValue

```js
var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var schema = new Schema({ title: String });
schema.path('title').get(function(v) { return v.toUpperCase(); })

var BlogPost = mongoose.model('BlogPost', schema);

var blogPost = new BlogPost()
blogPost.init({ title: 'Foo' })
blogPost.set({ title: 'Foo' })

// should be false 
console.log(blogPost.isModified('title'));
```