Formatter coerces types and bypasses request validation

[bplotnick]

When a param has a format, the formatter will coerce the value to that type instead of failing when validate_requests is enabled.

Pet Store example using bravado - petId has an int64 format:

petstore_json = requests.get('http://petstore.swagger.io/v2/swagger.json').json()
c = bravado.client.SwaggerClient.from_spec(spec_dict=petstore_json)
c.pet.getPetById(petId="123") # the string is coerced to a long
petstore_json['paths']['/pet/{petId}']['get']['parameters'][0].pop('format')
c = bravado.client.SwaggerClient.from_spec(spec_dict=petstore_json)
c.pet.getPetById(petId="123") # fails with ValidationError

This seems like a pretty serious issue as it causes unexpected behavior. At the very least, we should document it. I'm not entirely sure how we could fix this behavior at this point without breaking pretty much everyone.

Pinging the usual suspects: @sjaensch @macisamuele @analogue @jnb