Merge inline fragments whose type condition is the enclosing type

[antonshevchenko]

A type condition equal to the enclosing type is a no-op per the spec, but the codegen discards the fragment's selections.

When the fragment is the only selection, the result is an empty enum that can never deserialize:

query Example {
  thing {                 # returns the concrete object type `Thing`
    ... on Thing { name }
  }
}

generates pub enum ExampleThing {}, and every response fails with unknown variant `Thing`, there are no variants.

When the parent selects anything else, it compiles and the fields silently vanish:

query Example {
  thing {
    id
    ... on Thing { name }   # `name` is absent from the generated struct
  }
}

Fixes #394, #416.

[tomhoule]

Awesome! Thank you for the contribution.