Fix emails encoding to UTF-8 for html mails sent with the SendMail operation

Mails in html sent with the SendMail operation are not well encoded. At least, there is no information about which encoding to use so even if the content has the right encoding, the mail may not be display properly

Steps to reproduce:

• Create a chain with the Notification > Send E-mail operation
• tick html as parameter
• set message to *expr:@ {Document["note:note"]}

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• call the chain in Nuxeo Cap
• reading the mail with a non html webmail will have the special characters not well encoded