test: check against strings using period as delimiter

An actual live example of this is when using `protoc` and producing OpenApi2 with fully qualified names on.  See [here](grpc-ecosystem/grpc-gateway#2191 (comment)).  The names generated will be of the form `protoname.v1.ServiceName`.

Author: alexifm

end_to_end_tests/golden-record/my_test_api_client/models/__init__.py

@@ -23,6 +23,7 @@
 from .import_ import Import
 from .model_from_all_of import ModelFromAllOf
 from .model_name import ModelName
+from .model_reference_with_periods import ModelReferenceWithPeriods
 from .model_with_additional_properties_inlined import ModelWithAdditionalPropertiesInlined
 from .model_with_additional_properties_inlined_additional_property import (
     ModelWithAdditionalPropertiesInlinedAdditionalProperty,

end_to_end_tests/golden-record/my_test_api_client/models/model_reference_with_periods.py

@@ -0,0 +1,44 @@
+from typing import Any, Dict, List, Type, TypeVar
+
+import attr
+
+T = TypeVar("T", bound="ModelReferenceWithPeriods")
+
+
+@attr.s(auto_attribs=True)
+class ModelReferenceWithPeriods:
+    """A Model with periods in its reference"""
+
+    additional_properties: Dict[str, Any] = attr.ib(init=False, factory=dict)
+
+    def to_dict(self) -> Dict[str, Any]:
+
+        field_dict: Dict[str, Any] = {}
+        field_dict.update(self.additional_properties)
+        field_dict.update({})
+
+        return field_dict
+
+    @classmethod
+    def from_dict(cls: Type[T], src_dict: Dict[str, Any]) -> T:
+        d = src_dict.copy()
+        model_reference_with_periods = cls()
+
+        model_reference_with_periods.additional_properties = d
+        return model_reference_with_periods
+
+    @property
+    def additional_keys(self) -> List[str]:
+        return list(self.additional_properties.keys())
+
+    def __getitem__(self, key: str) -> Any:
+        return self.additional_properties[key]
+
+    def __setitem__(self, key: str, value: Any) -> None:
+        self.additional_properties[key] = value
+
+    def __delitem__(self, key: str) -> None:
+        del self.additional_properties[key]
+
+    def __contains__(self, key: str) -> bool:
+        return key in self.additional_properties