Suppose I have the following two models

```ruby
class Post
belongs_to :user
end

class User; end
```

If I try to `map` a criteria twice, the second time the eager loading is not respected. For example:

```
> crit = Post.limit(7).includes(:user)
> crit.map

{ |r| r.user.try(:id) }

;
```

Will generate the db calls

```
db.find | STARTED | {"find"=>"posts", "filter"=>{}, "limit"=>7}
db.find | SUCCEEDED | 0.001872s
db.killCursors | STARTED |

{"killCursors"=>"posts", "cursors"=>[817538945765]}

db.killCursors | SUCCEEDED | 0.000116s
db.find | STARTED | {"find"=>"user", "filter"=>{"_id"=>{"$in"=>[BSON::ObjectId('55e5974764643400033a0000'), BSON::ObjectId('55e597456464340003280000'), nil, BSON::ObjectId('55e597436464340003050000'), BSON::ObjectId('55e5974664643400032f0000'), BSON::ObjectId(...
db.find | SUCCEEDED | 0.00046s
```

But calling the previous line once more generates an N+1 query

```
> crit.map

{ |r| r.user.try(:id) }

;
```

```
db.find | STARTED | {"find"=>"posts", "filter"=>{}, "limit"=>7}
db.find | SUCCEEDED | 0.001755s
db.find | STARTED | {"find"=>"user", "filter"=>

{"_id"=>BSON::ObjectId('55e5974764643400033a0000')}

, "sort"=>{"_id"=>1}}
db.find | SUCCEEDED | 0.00036199999999999996s
db.find | STARTED | {"find"=>"user", "filter"=>

{"_id"=>BSON::ObjectId('55e5974764643400033a0000')}

, "sort"=>{"_id"=>1}}
db.find | SUCCEEDED | 0.000387s
db.find | STARTED | {"find"=>"user", "filter"=>

{"_id"=>BSON::ObjectId('55e597456464340003280000')}

, "sort"=>{"_id"=>1}}
db.find | SUCCEEDED | 0.00029299999999999997s
db.find | STARTED | {"find"=>"user", "filter"=>

{"_id"=>BSON::ObjectId('55e597436464340003050000')}

, "sort"=>{"_id"=>1}}
db.find | SUCCEEDED | 0.000312s
db.find | STARTED | {"find"=>"user", "filter"=>

{"_id"=>BSON::ObjectId('55e5974664643400032f0000')}

, "sort"=>{"_id"=>1}}
db.find | SUCCEEDED | 0.00035000000000000005s
db.find | STARTED | {"find"=>"user", "filter"=>

{"_id"=>BSON::ObjectId('55e5974764643400033a0000')}

, "sort"=>{"_id"=>1}}
db.find | SUCCEEDED | 0.000346s
db.find | STARTED | {"find"=>"user", "filter"=>

{"_id"=>BSON::ObjectId('55e5974564643400031f0000')}

, "sort"=>{"_id"=>1}}
db.find | SUCCEEDED | 0.0008539999999999999s
db.killCursors | STARTED |

{"killCursors"=>"posts", "cursors"=>[816157481622]}

db.killCursors | SUCCEEDED | 7.7e-05s
```