Enforcing JSON ARRAY for some XML nodes

[rimutaka]

If an array with a single value is serialized into XML and then converted into JSON it stops being an array and becomes a value or an object. Then it fails to deserialize into Vec or any other collection type. Examples:

<a>
    <b>1</b>
    <b>2</b>
</a>

becomes

"a": {
    "b": [1,2]
}

and can be deserialized into Vec<i64>, but

<a>
    <b>1</b>
</a>

becomes

"a": {
    "b": 1
}

and fails if we try to put it into the same Vec<i64> type.

Proposed solution

1. Add JsonType::Array to enforce an array on the mapped XML elements the same way we enforce other types

2. Allow mixing JsonType::Array with other types, e.g. JsonType::AlwaysString so that

   <a>
   <b>1</b>
   </a>

   becomes

   "a": { "b": [1] }

   with Infer or

   "a": { "b": ["1"] }

   with AlwaysString

3. This will be an extension to recently merged type enforcement, which is an optional feature and will affect existing implementations.

@AlecTroemel , Alec, do you have a minute to review this? Does it make sense? Am I on the right path here?