On registration page, link to login doesn't carry redirect URL

[sccr410]

If a plugin or theme adds the redirect_to variable to the registration page, then a user realizes they have an account and want to login - the link on the bottom of the registration form to "Login" does not pass the redirect_to variable. If set once, the redirect_to variable should stay for all links on the reg/login forms.

The same problem exists if you are on the login form but then click the register link.

Duplicate of #18743.