Tuples don't work properly

[csm10495]

Hey folks,

Say I have something like:

import dataclasses
from dataclasses_jsonschema import JsonSchemaMixin
from typing import Iterable, List, Set, Tuple, Union

@dataclasses.dataclass(frozen=True)
class Config(JsonSchemaMixin):
    tags: Tuple[Union[str, None]]

Config.from_dict({
    'tags': []
})

This leads to a raise like so:

ValidationError                           Traceback (most recent call last)
~site-packages/dataclasses_jsonschema/__init__.py in _validate(cls, data, validate_enums)
    488             else:
--> 489                 validate_func(data, cls.json_schema(validate_enums=validate_enums))
    490         except JsonSchemaValidationError as e:

~site-packages/jsonschema/validators.py in validate(instance, schema, cls, *args, **kwargs)
    933     if error is not None:
--> 934         raise error
    935

ValidationError: [] is too short

Failed validating 'minItems' in schema['properties']['tags']:
    {'items': [{'type': 'string'}],
     'maxItems': 1,
     'minItems': 1,
     'type': 'array'}

On instance['tags']:
    []

The above exception was the direct cause of the following exception:

ValidationError                           Traceback (most recent call last)
<ipython-input-3-400dc9903cd4> in <module>
      8
      9 Config.from_dict({
---> 10     'tags': []
     11 })

~site-packages/dataclasses_jsonschema/__init__.py in from_dict(cls, data, validate, validate_enums)
    506         non_init_values: Dict[str, Any] = {}
    507         if validate:
--> 508             cls._validate(data, validate_enums)
    509
    510         for f in cls._get_fields():

~site-packages/dataclasses_jsonschema/__init__.py in _validate(cls, data, validate_enums)
    489                 validate_func(data, cls.json_schema(validate_enums=validate_enums))
    490         except JsonSchemaValidationError as e:
--> 491             raise ValidationError(str(e)) from e
    492
    493     @classmethod

ValidationError: [] is too short

Failed validating 'minItems' in schema['properties']['tags']:
    {'items': [{'type': 'string'}],
     'maxItems': 1,
     'minItems': 1,
     'type': 'array'}

On instance['tags']:
    []

Using a list tends to pass validation (but then messes with the hashing since the are mutable)