Allow the case of post object to be interrupted as an array value

[mboswell-kp]

A field of type post_object with the attribute multiple set to true was yielding incorrect data results.

Example JSON for fields.json

{
              "label": "Foobar",
              "name": "foobar",
              "type": "post_object",
              "required": 0,
              "conditional_logic": 0,
              "wrapper": {
                "width": "",
                "class": "",
                "id": ""
              },
              "post_type": [
                "foos"
              ],
              "taxonomy": "",
              "allow_null": 0,
              "multiple": 1,
              "return_format": "object",
              "ui": 1
}

With multiple posts saved in the acf field the value sent to parse_acf_field would have the following shape:

[array]
 - WP_Post
 - WP_Post
 - WP_Post
 - ...

A call to get_post with this array value always returns the first post as WordPress's native code casts it to an integer.

This fix should allow the post object to be able to handle an attribute of multiple thus serializing multiple post objects.