Allow omitting Directive arguments that are non-null if they have defaults

[StevenACoffman]

Allow omitting Directive arguments that are Non-null AND have defaults.

Fix #269

Follow-up to #258 : Directives should validate argument names and non-null fields.

Minimal graphql.schema and models to reproduce

type AircraftManufacturer
  @join__type(graph: AIRCRAFT, key: "uuid")
{
  uuid: ID!
  name: String!
}

The directive definition of:

directive @join__type(
  graph: Graph!,
  key: FieldSet,
  extension: Boolean = false,
  resolvable: Boolean = true,
  isInterfaceObject: Boolean = false
) repeatable on OBJECT | INTERFACE | UNION | ENUM | INPUT_OBJECT | SCALAR

so the argument in question (extension) is non-nullable but has a default so still isn't required.

Signed-off-by: Steve Coffman [email protected]

[coveralls]

coverage: 88.607%. remained the same when pulling c1fecac on allow_directive_arg_empty_nonnull_if_default into 4b44f9b on master.