noirbizarre / flask-restplus / 793

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Fix ambiguity error with Polymorphism candidates (isinstance() problem) (#691)

* Fix for better Polymorphism support

Right now Polymorphism fails when using something like this (models in sqlalchemy):

models:

```python
class Product(BaseModel):
	discr = Column(String)

	__mapper_args__ = {
		'polymorphic_on': discr,
        'polymorphic_identity': "product"
	}

class VegetableProduct(Product):
	name = Column(String)

    __mapper_args__ = {
		'polymorphic_identity': "vegetable"
	}

class DairyProduct(VegetableProduct):
    __mapper_args__ = {
		'polymorphic_identity': "dairy"
	}
```

mapper:

```python
resource_product = api.model("Product", {
	'discr': fields.String
})

product_mapper = {
	VegetableProduct: api.inherit("Vegetable", resource_product, { 'name': fields.String }),
	DairyProduct: api.inherit("Dairy", resource_product, { 'name': fields.String }),
}

resource_result = api.model("ResourceResult", {
	'products': fields.List(fields.Polymorphism(product_mapper))
})
```

This sparkles error here:

https://github.com/noirbizarre/flask-restplus/blob/master/flask_restplus/fields.py#L682

Because candidates will be: VegetableProduct and DairyProduct, since isinstance() will return True for both classes (I personally surprised it works like this)

Checking by __class__.__name__ removes this error and it works as intended.

I hope my quick fix did not break anything, I'm eager to update it if I don't know, more elegant solution is needed.

* adjusted ambiguity test for new feature

* stricter check for type

* changed to simple class check

* type() check for Polymorphism, adjusted tests

* type() check for Polymorphism, adjusted tests

* removed local dev files

* updated test name, also PR in docblock

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