ensuring 'workspace_id' is passed as int, during invite-processing

[pazzernick]

Fixes #164

Prior to fix:

• registration process for a new user fails, with PHP Error noted in Error when registering user on invite #164
• it appears the 'workspace_id' returned from database model is a valid workspace number, but not an integer

After fix:

• the 'workspace_id' is now converted to integer type, and passed to function which expects integer
• invitation processing works, new user can now register and proceed to 'email validation' stage

[JonoB]

Thank you for the PR!