Map keys using custom types not being serialized with `MarshalText`

[styeung]

Assuming jsoniter is supposed to be 100% compatible with encoding/json, it appears that jsoniter does not serialize map keys of custom types correctly, in that it does not use the MarshalText method when provided.

The standard behavior is that any type implementing the TextMarshaler interface should use MarshalText when marshaling itself.

The strange thing is that this incorrect behavior only happens when the map key is a value. When the map key is a pointer, it's fine.

To illustrate what I mean, consider these four scenarios:

• using encoding/json - key is pointer
• using encoding/json - key is value
• using jsoniter - key is pointer
• using jsoniter - key is value

You'll notice that the first three scenarios is able to pick up on the custom behavior added by MarshalText, but the fourth one does not.