Support toDocument from array data alongside json data

[danbelden]

Currently the handler only supports converting to document from json data.

This means you need to encode the array only to have it decoded when converting to document.

• https://github.com/danbelden/php-siren/blob/master/src/Handler.php#L32

It would be nice if the handler supported a toDocumentFromArray() method:

public function toDocumentFromArray(array $array)
{
    // Code to return siren document
}