Don't use an `NSNumber` key in a dictionary which will be saved as a preference,...

Don't use an `NSNumber` key in a dictionary which will be saved as a preference, as this prevents the entire preference group from saving; you must use an `NSString`. We now have logging which points the problem out.

Fixes #9082 (Custom status pre-population with previous message.  Fixes #225 (yes, a ticket from 3 years ago!) which was previously very, very mysterious - the preference in question is in the same group as the last-status preference, which is why it wouldn't save.

git-svn-id: svn://svn.adiumx.com/adium/trunk@22539 1c916fce-81e2-0310-a464-8ca513f45935