Prefer variable key from examples when possible

[kylef]

We can extract the key to use for variable member derived from Schema Object additionalProperties from an example, providing the key is not already used for a property (with different schema) or has different require-ness constraints.

The change makes it so that the example in the following OpenAPI 2 Schema Object is not discarded when it matches the appropriate type, it is instead in the parse result:

type: 'object',
additionalProperties: {
  type: 'string',
},
example: {
  'application/json': 'https://tools.ietf.org/html/rfc7159',
},

[kylef]

I was trying to see if we could "move" the example value here too, but it does get tricky quick due to the complexities when the schema for the additional properties could contain various constraints (fixed, fixedType, be a container, etc) which change what the "value" means.

I'll perhaps follow up with that when I get a chance.