[FIX] email_template: support of email_recipients in send_mail.
email_recipients is a char fields holding a comma separated list of partner_ids
that are the email recipients. This field is correctly handled in the composer
but was not taken into account in the send_mail method email_template.
However due to model limitations, this field can only be taken into account
when sending the email directly using force_send. When the email is
queued, no field is available to hold the value of email_recipients.
This is already fixed in trunk/v8 as a field has been added to handle
this value.

[FIX] mail: when sending email to some recipients_ids, filter to have only
the existing partner_ids. Trusting email template to generate valid
partner_ids is a bit risky.

[TESTS] email_template: added tests for send_mail