Support for default value for union of records

Nobody has claimed this yet.

We have a schema where one of the attributes is a union of two record types:

```
{
"type": "record",
"name": "Thing",
"namespace": "universe.of.things",
"fields": [
{
"name": "layout",
"type": [
{
"type": "record",
"name": "AnotherThing",
"namespace": "another.universe.of.things",
"fields": [
{
"name": "text",
"type": "string",
"default": "someText"
}
]
},
{
"type": "record",
"name": "AnotherThing2",
"namespace": "another.universe.of.things",
"fields": [
{
"name": "text",
"type": "string",
"default": "someOtherText"
}
]
}
],
"default": {
"another.universe.of.things.AnotherThing": {
"text": "someDefaultText"
}
}
}
]
}
```

When putting it through `NewCodec` we get an error: `Record "universe.of.things.Thing" field "layout": default value ought to encode using field schema: cannot encode binary union: no member schema types support datum: allowed types: [another.universe.of.things.AnotherThing another.universe.of.things.AnotherThing2]; received: map[string]interface {}`

I created a PR #123 which works fine for us, and is what we're using for now.

Start with NewCodec and the supplied union-of-record schema, then compare the current behavior with PR #123, which the report says already works for this case. Done means the default value for the union of records is accepted without the reported encoding error and existing behavior remains intact.

Written by the indexing model from the issue text.