Add merge_field support to embedded template editing

[djhype]

Per the API documentation entry for embedded templates (https://app.hellosign.com/api/embeddedTemplatesWalkthrough#EmbeddedTemplatesEditingATemplate), the API supports passing a merge_fields property when retrieving a embedded template's edit_url. However, adding merge fields to the Template object and passing the Template object to the Client::getEmbeddedEditUrl() method did not result in a change to the template's merge fields. This PR adds support for changing merge_fields when updating an embedded template.

The merge_fields change act as an atomic update in that ALL fields (both existing and new) must be included in the merge_fields JSON string. This appears to be the intended functionality as I experienced this when issuing a cURL command directly to the API.

According to PHPStorm, the only usages of the Template::toParams() method is in the Client::getEmbeddedEditUrl() method so this likely has low-to-no impact anywhere else. The code snippet is exactly the same as the handling of the Template::merge_fields property in the Template::toEmbeddedDraftParams() method. Thus, the code isn't very DRY. However, all of the toXxParams() methods in the Template class aren't very dry. If you like, I will amend this PR so that the various toXxParams() methods all use a single method to generate the valid parameter array.

[OnionKnightN]

Thank you for submitting this pull request! We are having this reviewed internally and will write back to you on the update of this fix as soon as possible.

[CLAassistant]

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[jtreminio-dropbox]

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