custom validation on embedded documents

Hi,

I wrote a custom validation on a embedded document e.g.

``` javascript
var sub = new Schema({
content: String
});

sub.path('content').validate(function (value) {
// do something
});

var schema = new Schema({
title: String,
body: String,
subDoc: [sub]
});

var myModel = new Model('MyModel', schema);
```

In the validation callback I can access `this`. If I validate like example no. 1 `this` will be the document itself. If I validate like example no. 2 `this` will be the embedded document.

Example 1

``` javascript
var doc = new myModel({
title: 'hello',
body: 'world',
subDoc: [{
content: 'hey!'
}]
});

doc.validate(function () {});
```

Example 2

``` javascript
var doc = new myModel({
title: 'hello',
body: 'world',
subDoc: [{
content: 'hey!'
}]
});

doc.subDoc[0].validate(function () {});
```

I found in `lib/document.js` in the `validate` ([master](https://github.com/LearnBoost/mongoose/blob/master/lib/document.js#L1024-L1035), [3.8.x](https://github.com/LearnBoost/mongoose/blob/3.8.x/lib/document.js#L969-L986)) method executen of the valdators.

``` javascript
process.nextTick(function(){
var p = self.schema.path(path);
if (!p) return --total || complete();

var val = self.getValue(path);
p.doValidate(val, function (err) {
if (err) {
self.invalidate(path, err, undefined, true);
}
--total || complete();
}, self);
});
```

I forked the repository, modified the code and run the tests.

``` javascript
process.nextTick(function(){
var p = self.schema.path(path);
if (!p) return --total || complete();

var val = self.getValue(path);
var parts = path.split('.');
parts.pop();
p.doValidate(val, function (err) {
if (err) {
self.invalidate(
path
, err
, undefined
, true // embedded docs
);
}
--total || complete();
}, parts.length ? self.getValue(parts.join('.')) : self);
});
}
```

Now `this` in the validation callback is everytime the embedded document, but I don't know if the current behavior is intended. I can make a pull request, but I want to ask before.

(Sorry for my bad english :smile:)

Start in lib/document.js at the validate method and compare the callback context in the two embedded-document examples. Review the validation tests the reporter ran, then determine whether callbacks should receive the parent document or embedded document and add regression coverage for the intended behavior.

Written by the indexing model from the issue text.