[FIX] mass_mailing: restore save on blur

Commit [1] defined a value for the `onWysiwygBlur` option of `Wysiwyg`
in the `mass_mailing` `html field`. This had the side effect of
overriding the value set by its parent `html field`, which was to call
`commitChanges`. As a result, the `mass_mailing` `html field` was not
saving its content on blur. This restores that behavior by explicitly
adding the call to `commitChanges` where it was missing.

[1]: https://github.com/odoo/odoo/commit/e212e873eb35eb9b9a931fb1521109475c008074



task-3288881

closes odoo/odoo#120113

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>