Fix form multiselect request validation

Author: PierrickBrun

src/quart_schema/validation.py

@@ -145,7 +145,10 @@ async def wrapper(*args: Any, **kwargs: Any) -> Any:
             if source == DataSource.JSON:
                 data = await request.get_json()
             else:
-                data = (await request.form).to_dict()
+                data = (await request.form).to_dict(flat=False)
+                for key, value in data.items():
+                    if len(value) == 1:
+                        data[key] = value[0]
                 if source == DataSource.FORM_MULTIPART:
                     files = await request.files
                     for key in files: