Email handler body translation broken

Problem/Motivation

When translating a web form with an email handler using a custom body, the translation of the body goes assigned to the "_default" string in user translate interface even if the option of the email body is to use a custom body. This can cause problems if you have multiple webforms translated with custom body because the "_default" string will receive the latest saved webform message and when the translation is updated during a module install or locale-update commands all webforms will get same message as the latest one saved.

Steps to reproduce

You can reproduce it also with the templates provided by default by Webform. Let's take as example "Subscribe" template

You need:
1. Webform latest version
2. A second language so you can translate the webform.

1. Go to User translation interface and search for the string _default -> should be empty or _default
2. Go to Webform Templates -> Click Manage -> Select "Subscribe" webform and go to the email handlers.
3. Change the body to use "Custom body" and add a message "Test message"
4. Translate the template webform and just save the new translation created.
5. Go back to User translation interface and search for string _default -> it contains the text from point 3.