[FIX] mail: thread: keep image ratio
[FIX] mail: thread: keep image ratio

With rule 'max-width: 100%', the height must be reset to 'auto' to keep the
image ratio.

ygol/odoo,hip-odoo/odoo,dfang/odoo,dfang/odoo,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,dfang/odoo,dfang/odoo,hip-odoo/odoo,dfang/odoo,hip-odoo/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo,dfang/odoo