Allow MapAttribute instances to be used as the RHS in expressions.

[jpinner-lyft]

The expression classes assume that whenever an attribute appears in the right hand side argument that the resulting expression should use the attribute's path. That allows users to compare two attributes of an item or to update one attribute of the item based on another. For example:

>>> class MyModel(Model):
...     foo = UnicodeAttribute()
...     bar = UnicodeAttribute()
...
>>> MyModel.foo > MyModel.bar
foo > bar
>>> MyModel.foo.set(MyModel.bar)
foo = bar

However, the code failed to consider that some attribute classes, specifically MapAttribute and its subclasses, can sometime act as AttributeContainer objects depending on the context. When instantiated as an attribute container, the instance should behave as a Value in the right hand side of an expression:

>>> class MyMap(MapAttribute):
...     foo = UnicodeAttribute()
...
>>> class MyModel(Model):
...     my_map = MyMap()
...
>>> my_map_instance = MyMap(foo='bar')
>>> MyModel.my_map.set(my_map_instance)
my_map = {'M': {'foo': {'S': u'bar'}}}