Clear ViewContext before specs instead of after

By doing it before, we get sure that another type of spec (model,
acceptance) does not leave ViewContext in a wrong context...

https://github.com/drapergem/draper/issues/523
Merge pull request #547 from gaizka/master

Clear ViewContext before specs instead of after
Alias `object` to `#{model_name}`

If we have an `ArticleDecorator` there is going to be a method
added to it named `article` which is convenient for the user.
Require NameError#missing_name? from ActiveSupport
Improve aliasing to object class name
Only define MongoidPostDecorator when using Mongoid