Fix no URL validation for ModelSchema URLFields

[lapinvert]

Fixes #1157

Since Django's implementation of of get_internal_type() returns 'CharField' for URLField, we were not able to assign a more appropriate Pydantic type to it, such as AnyUrl. As a result, there was no validation for URLs in post requests that expected a ModelSchema using URLFields.

This PR fixes this by adding a special case for 'URLField' to check it's object type with and assign an internal_type of 'URLField'.

Related tests were also updated.

[lapinvert]

I added a commit because the previous one would break JSON serialization, since AnyUrl doesn't serialize as a string. It would throw Object of type Url is not JSON serializable.

The problem was discussed over here in Pydantic github and I implemented one of the solutions suggested there by pavradev and updated the tests.

[lapinvert]

I had to improve on the previous solution which would throw Input should be a valid string as it would appear that validation on individual objects is done (= when response=PostSchema) whereas it's not done when returning lists of objects (response=List[PostSchema]).