JSON Marshalling Of Contains

[RichardKnop]

When I unmarshall a schema with contains keyword into instance of *jsonschema.Schema, for example:

{
  "contains": {
    "const": "foobar"
  },
  "type": "array"
}

And then marshall the schema instance back into JSON, I expect:

{
  "contains": {
    "const": "foobar"
  },
  "type": "array"
}

What I actually get is:

{
  "contains": {},
  "type": "array"
}

Here is a fix: #125