why isnt there an option for a default enum so that the writer can add new enum symbols

[Jim Donofrio]

The schema resolution page says:

> if both are enums:
> if the writer's symbol is not present in the reader's enum, then an 
error is signalled.

Is there a reason you could not allow us to provide one of the symbols 
as a default in the reader so that when the reader read the enum with 
symbol it didnt have, that new symbol would get defaulted to the default 
symbol we provide?

For example with the below schema's this would currently fail if the old 
reader encountered a Enum1.C in the data. Why not provide users the 
option to set "default": "A" for example so that any unknown enum's are 
bined into one? The user might have an unknown enum or something. 
Currently I was going to implement this in my application by just using 
an int type and looking up the ordinal of my own Java Enum. If the int 
is larger than any of the oridinals, the enum would be set to my own 
default value in the enum.

Thanks.

Original schema:

{
    "type": "record",
    "name": "EnumExample",
    "fields": [
        {
            "name": "enum1",
            "type": {
                "type": "enum",
                "name": "Enum1",
                "symbols": ["A", "B"]
            }
        }
    ]
}

New schema:

{
    "type": "record",
    "name": "EnumExample",
    "fields": [
        {
            "name": "enum1",
            "type": {
                "type": "enum",
                "name": "Enum1",
                "symbols": ["A", "B", "C"]
            }
        }
    ]
}