Omittable can now be serialized as json

[endSly]

The Omittable struct lacks the necessary JSON unmarshaller and marshaller implementations. As a result, instances of this struct are serialized as empty JSON objects ({}) even when they contain data.

For instance, consider the following Go struct:

struct {
	ID     graphql.Omittable[*string]     `json:"id,omitempty"`
	Bool   graphql.Omittable[*bool]       `json:"bool,omitempty"`
	Str    graphql.Omittable[*string]     `json:"str,omitempty"`
}

This struct is being serialized as:

{
  "id": {},
  "bool": {},
  "str": {},
}

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

coverage: 76.16% (+0.1%) from 76.039%
when pulling 302dbc8 on endSly:omittable-json
into 132ec1c on 99designs:master.

[StevenACoffman]

Thanks! I appreciate you adding this, and I'm looking forward to more PRs from you!