don't include associated models in to_params

Let's assume the following models:

```ruby
class User
  include Her::Model
  has_many :blog_posts
end

class BlogPost
  include Her::Model
end
```

Now when I consume an API which includes the corresponding blog posts in the user response, I get an object like this:

```
Foo::User.find(1)
> #<Foo::User(users/1) id=1 name="Tobias Fünke" blog_posts=#<Her::Model::Associations::HasManyAssociation:0x007ff6aa2e9d68 @parent=#<Foo::User(users/1) id=1 name="Tobias Fünke" blog_posts=#<Her::Model::Associations::HasManyAssociation:0x007ff6aa2e9d68 ...>>, @opts={:class_name=>"BlogPost", :name=>:blog_posts, :data_key=>:blog_posts, :default=>[], :path=>"/blog_posts", :inverse_of=>nil}, @params={}, @klass=Foo::BlogPost, @name=:blog_posts>>
```

When trying to save this model, it sends the following parameters:

````
{:id=>1, :name=>"Tobias Fünke", :blog_posts=>[#<Foo::BlogPost(blog_posts/1) id=1 title="Welcome To codinghell.ch" user_id=1>, #<Foo::BlogPost(blog_posts/2) id=2 title="How Awesome Is This" user_id=1>]}
```

This doesn't seem correct to me. This PR changes the to_params method in `/lib/her/model/parse.rb` to exclude associated models from parameters altogether.