Fix handling of array/object enum samples in JSON 0.6 serialisation

[kylef]

Given the following API Element:

{
  "element": "enum",
  "meta": {
    "id": {
      "element": "string",
      "content": "Schema"
    }
  },
  "attributes": {
    "enumerations": {
      "element": "array",
      "content": [
        {
          "element": "string"
        },
        {
          "element": "number"
        },
        {
          "element": "boolean"
        },
        {
          "element": "object"
        },
        {
          "element": "array"
        }
      ]
    },
    "samples": {
      "element": "array",
      "content": [
        {
          "element": "object",
          "content": [
            {
              "element": "member",
              "content": {
                "key": {
                  "element": "string",
                  "content": "message"
                },
                "value": {
                  "element": "string",
                  "content": "Hello World"
                }
              }
            }
          ]
        }
      ]
    }
  }
}

Which is derved from OAS 3 document schema of:

Schema:
  example:
    message: 'Hello World'

Since the schema isn't limited to any type, it's effectively an "enum" of all types, with an example value for an object with values.

This would previously raise the following exception:

TypeError: Cannot read property 'remove' of undefined
    at samples.forEach (/Users/kyle/Projects/apiaryio/api-elements.js/node_modules/minim/lib/serialisers/JSON06Serialiser.js:125:33)
    at content.forEach (/Users/kyle/Projects/apiaryio/api-elements.js/node_modules/minim/lib/primitives/ArrayElement.js:174:29)
    at Array.forEach (<anonymous>)
    at ArrayElement.forEach (/Users/kyle/Projects/apiaryio/api-elements.js/node_modules/minim/lib/primitives/ArrayElement.js:173:18)
    at JSON06Serialiser.enumSerialiseAttributes (/Users/kyle/Projects/apiaryio/api-elements.js/node_modules/minim/lib/serialisers/JSON06Serialiser.js:124:13)
    at JSON06Serialiser.serialise (/Users/kyle/Projects/apiaryio/api-elements.js/node_modules/minim/lib/serialisers/JSON06Serialiser.js:25:31)
    at JSON06Serialiser.serialiseContent (/Users/kyle/Projects/apiaryio/api-elements.js/node_modules/minim/lib/serialisers/JSON06Serialiser.js:281:19)
    at JSON06Serialiser.dataStructureSerialiseContent (/Users/kyle/Projects/apiaryio/api-elements.js/node_modules/minim/lib/serialisers/JSON06Serialiser.js:105:18)
    at JSON06Serialiser.serialise (/Users/kyle/Projects/apiaryio/api-elements.js/node_modules/minim/lib/serialisers/JSON06Serialiser.js:53:67)
    at Array.map (<anonymous>)

[kylef]

This is the fix for the particular exception, the "content" of the sample (object) is an array in this case and thus does not have "attributes" (which returns undefined). The fix here is adding a "cheap" check if the sample's content exists, and it is an element.

[kylef]

Once I fixed the exception, I discovered the output from the serialiser to be incorrect, we was taking the members (sample.content) and wrapping them in an array. Instead I've added a check that if sample is already an array type (such as an object element) then we will just wrap the entire element in an array and not it's content (element).