remove defaults in deps and add .as_dict method to the DefaultDependency class

[vincentsarago]

This PR does:

• remove defaults to most of dependencies (and default to rio-tiler values)
• In order to defaults to rio-tiler's default we have to NOT pass options to rio-tiler's method. when we use Dependency unpacking we would pass value=None, to overcome this, we introduce a new .as_dict() method which will return a Dict from the dependency but only with non-None values