Attachments should not persist across calls to send()
Allow callers of EDD_Emails->send() to specify the main heading that goes in the email template
Fixes to previous attachment changes
Merge branch 'attachments_persistence' of
https://github.com/leewillis77/Easy-Digital-Downloads into
leewillis77-attachments_persistence
Remove changes relating to heading in email template
Merge branch 'attachments_persistence' of github.com:leewillis77/Easy-Digital-Downloads into leewillis77-attachments_persistence
Fix merge conflict for #2551