CRM-19690 - Mailing API - Encode and decode `template_options`

CRM-19690 - Mailing API - Encode and decode `template_options`

Present API consumers with a consistent, array-based interface for reading
and writing properties in a Mailing.

Suppose you're submitting a REST request to create a mailing.  The REST
request as a whole is encoded with JSON.  With the default API behavior, you
would need to double-encode the `template_options`, e.g. roughly

```
POST rest.php
  json_encode([
    entity => Mailing
    action => create
    params => [
      template_options => json_encode([...])
    ]
  ])
```

With this patch, you only need to encode the request once.

This parallels the approach used in CaseType API (for the XML `definition`
field).