Custom Enum Codec is not called in most situations

[eheimburg]

Describe the bug
It's not possible to use a custom Codec for an enum. The problem is that the built-in converters are checked first. There's a line in Conversions.java where it converts an enum to a string, bypassing the codecs.

To Reproduce
I've created a reproducer here: https://github.com/eheimburg/enumserializedbug

It includes a custom codec factory and a custom codec for an enum. The factory prints when it's called, so it's easy to see that the factory isn't called for the needed codec.

Please complete the following information:

• Server Version:
• Driver Version:
• Morphia Version: 2.3.0

Additional Info
I was able to work around this issue by creating a custom Converter for my enum. So this is not blocking me, but it does seem weird and surprising.

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My goal in creating the custom enum codec is to handle invalid values better (defaulting to e.g. INVALID instead of throwing an exception). If the stock converter is important for performance reasons, I'd be happy with any solution that lets me handle invalid enum values.

[eheimburg]

Oh, and for quick access: I believe the culprit is in dev.morphia.mapping.codec.Conversions in convert() at line 150:

            if (target.equals(String.class)) {
                return (T) value.toString();
            }
            if (target.isEnum() && fromType.equals(String.class)) {
                return (T) Enum.valueOf((Class<? extends Enum>) target, (String) value);
            }

Since all enums are converted as primitives, there's never an attempt to find a codec.

(This means the built-in EnumCodec is largely unused also, from what I can tell.)

[evanchooly]

enums have long been a problem because it seems the driver's codec usually gets in the way. I thought I had this fixed way back but it seems there's a bit more work to do.