Validate correct use of traits

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:

```yaml
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):

```yaml
/{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:

```yaml
/{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.