Cannot subscribe by email to tags with spaces

[solarissmoke]

Migrating from ​#wp17595:

To reproduce:

• Visit ​http://wordpress.org/tags/disable-comments when logged in
• Click on "Subscribe to Emails for this Tag"

After reload the link should now read "Unubscribe to Emails for this Tag", but it hasn't changed, and the user does not receive emails.

The problem appears to be when people use tags that include a space, i.e., "disable comments".

[ocean90]

Fixed in [3719].