…se support

…percased

Some helpers to deal with converting auto-generated codable String values for instance variables to match some common XML key coding standards to the commonly used Swift camel casing

- capitalzied: convert first letter to uppercase
- uppercased: All letters uppercased
- lowercased: All letters lowercased

Support all types that conform to StringProtocol rather than just String

Use a type erased protocl inheritance strategy commonly used to provide default implimentation to avaoid issues with as? checks in generic protocols, while retaining reuse benefits of generic protocols

…y string

In the case where a codable provides an empty string for the codable string value for an instance variable an empty bracket was inserted which is invalid XML.

```
let attr = "bar"
let value = "FOO"
enum CodingKeys : String, CodingKey {
    case attr
    case value = ""
}
```

Will be useful for unkeyed objects that contain only attributes eg;
```xml
<box attr="bar"><>FOO</></box>
<!-- Would now correctly become -->
<box attr="bar">FOO</box>
```

DynamicNodeEncoding allows easily adding the ability to choose if iVars should be attribute or element encoded by inheriting DynamicNodeEncoding and implimenting a single static function in any Codable class or struct.

This is simpler than the current method that requires a global dynamic encoding closure for every XMLEncoder instance. This allows changing the encoding where the data models live, rather than the creator of the XMLEncoder instance needing to have knowledge of all the possible structs and classes that the encoder might encounter at init time.

- refactor element and attribute encoders to closures for easier code reuse
- Added type alias for encoding closures for clarity

Had removed them since I was testing intrinsics with attributes. Needed to wrap cateogy value in an element tag again.

Also appears the hack for decoding nested arrays is no longer required so removed the complex decoding of Category

Previous version of this test techncially passed on Encdode/Decode comparision sinve the structure values were the same, but the encoding make Book structs id an element, so the strings weren't equal.

Modified the simplier single book test to check that the attributes are encoded to XML and match the original string (minus white space formatting)