"fixed_version_id" - "value" is not string type, but int type

[sanak]

Problem
This is the cause of the following error.
hyakumori/smash#36

Reported by @mbasa

{
    "key": "fixed_version_id",
    "label": "Version",
    "values": {
        "items": [
            {
                "item": {
                    "label": "Test Milestone",
                    "value": "2"
                }
            }
        ]
    },
    "value": 2,
    "type": "stringcombo"
}

To Reproduce
Call project level API endpoint.
http://localhost:3000/projects/(project_id)/smash/tags.json

Expectation
The value should be string type.