Required fields for campaign creation

[elisaprio]

Hello,
First, thanks for your work on this package.

I noticed today that to created a campaign, reply_to, from_name and subject_line are not required, and settings neither.
And subject_lines, from_names, reply_to_addresses are required for a campaign of type 'variate'. If they are not filled, it falls back to settings and reply_to, from_name and subject_line.
I created a pull request for this : PR

thanks,
Elisa