Validate correct use of traits

[stmeissner]

It looks like the RAML validator has issues when traits are used at the wrong place in the API spec.
When defining this trait for a HTTP method header:

traits:
  right-header:
    headers:
      Accept:
        displayName: Accept Header
        type: string
        description: Accept Header
        required: true
        pattern: ^application/json$
        default: application/json

and using it at a wrong place in the API spec (the header must be on the method):

/{identifier}:
  uriParameters:
    identifier:
      displayName: DataSource Identifier
      type: string
      description: The data source identifier.
  is:
    - right-header
  get:
  ...

Actual behavior

The RAML validator does not complain.

When I model the same explicitly without the trait:

/{identifier}:
  uriParameters:
    identifier:
      displayName: DataSource Identifier
      type: string
      description: The data source identifier.
  headers:
    Accept:
      displayName: Accept Header
      type: string
      description: Accept Header
      required: true
      pattern: ^application/json$
      default: application/json
  get:
  ...

the RAML validator throws an error as expected.

Expected behavior

The RAML validator throws an error when using the trait at a place where its content is not compliant to the RAML standard.