Fix bug that would overwrite the content of the response body when using multiple presenters

When using multiple presenters that make use of Grape Entity, if the user does not specify the root when calling the present method, the contents added to the body by the first presenter would be overwritten by the second presenter. 

This issue becomes apparent when the user does not pass a root to the present method but instead opts to use the 'root' inside the Grape Entity being provided to present method. 
This is mutually exclusive with above, change it to be something like 

```ruby
if key
   # ...
elsif ...
   # ...
end
```