fix: enum schema evolution

[redaLaanait]

fixes: #340

[nrwiersma]

Looks good, perhaps a test to prove it will substitute the default when the writers symbol is missing, stopping any regression here?

[redaLaanait]

I think this one does cover the case:

 {
    name: "Enum Reader Missing Symbols With Default",
    reader: `{
        "type": "enum",
        "name": "test.enum",
        "symbols": ["foo"],
        "default": "foo"
    }`,
    writer: `{
        "type": "enum",
        "name": "test.enum",
        "symbols": ["foo", "bar"]
    }`,
    value: "bar",
    want:  "foo",
}

Do I understand the remark correctly?

[nrwiersma]

You are correct, I missed that this does an Unmarshal.

[nrwiersma]

LGTM 🎉