[FIX] sale: do not propagate context on send email

Otherwise, active_id, active_ids, active_model params are propagated, and it leads to inconsistencies.
For instances, go to Sales > Customers, choose a customer which has quotations.
On the form, hit the "Quotations and sales" Button
Choose a quotation. Hit the "send by email" button.
The template preview should be displayed correctly. Now send the email, than
 - Either, you have no traceback: The email sent is the email of another quotation, the quotation having as id the id of the partner you chose
 - Either, you have a traceback: There is no quotation having as id the id of the partner you chose.