Implicit render should set actual content type

Implicit render is not setting actual content type if there's been any other format rendered.
For instance:
```ruby
def action
  string = render_to_string :template =>"template", :formats => [:js]
end
```
with test 
````ruby
test "content_type"
  get :action
  assert_equal "text/html", response.content_type
end
```
will set `Content-Type="text/javascript; charset=utf-8"` in header, even though html is processed and rendered.