Fix dynamic embedded document updates

[mcsimps2]

In order to allow setting new fields that did not previouly exist on dynamic embedded documents, the lookup_member functions on EmbeddedDocumentField and GenericEmbeddedDocumentField return dynamic fields as appropriate. Special awareness is given to positional operators - see the new test cases for examples of where this is important.

Here's an example of what this now allows

class Wheel(DynamicEmbeddedDocument): position = StringField() class Car(Document): wheels = EmbeddedDocumentListField(Wheel) # This is now allowed - adding a new field to the dynamic embedded document Car.objects(wheels__position="front-driver").update( set__wheels__S__damaged=True ) 

Resolves #2486