added functionality to exclude columns after serialzation from to dict

[PrasanthChettri]

columns can be added in exclude list in the models and when we call to to_dict from the serializer mixin it filters out those columns

[michaelbukachi]

Hey @PrasanthChettri thanks for the PR!. Do you mind moving the __exclude__ from the model to the to_dict method. Maybe something like:

def to_dict(self, nested=False, hybrid_attributes=False, exclude=None):

This way, users won't have to touch their models.

Also, don't forget to add tests for the new functionality :smile:

[PrasanthChettri]

Yeah sure will do that

[PrasanthChettri]

I rewrote existing tests to accommodate the new change instead of adding new tests, hope that was fine