Support 'merge' parameter in updateEmail API call

[MichaelOrmondeHostelworld]

🔹 Jira Ticket(s)

• None: is support for an endpoint parameter based on ongoing Iterable implementation

✏️ Description

• Add support for the "merge" parameter in the https://api.iterable.com/api/docs#users_updateEmail API endpoint
• This parameter is not mentioned in the documentation link above, but is available on the endpoint upon activation
• Retain support for func calls without "merge" parameter for both backwards compatibility and Obj-C interoperability
• Relevant unit tests added