Handling inline references in the main schema even when subSchemas are provided

[bardzusny]

Right now, if subSchemas are provided, it prevents inline references in the main schema from being resolved.

javascript
var schema = Enjoi({
'title': 'Example Schema',
'type': 'object',
'properties': {
'firstname': {
'$ref': '#/definitions/firstname'
},
'surname': {
'$ref': 'definitions#/surname'
}
},
'definitions': {
'firstname': {
'type': 'string'
}
}
}, {
'definitions': {
'surname': {
'type': 'string'
}
}
});

Would throw up this error:

AssertionError: Can not find schema reference: #/definitions/firstname.

This PR fixes just that + adds proper test checking this use case.

(related: https://github.com/krakenjs/swaggerize-builder/issues/33 ).

[bardzusny]

I believe this was resolved by 31175d9 (allowing to override source for inline references by providing subSchema with # key). Also: see #7