Use an appropriate name in the Reply-To header

Use "Site Name <>" for the Reply-To header when the reply is to the site or a public topic.

Use "username <>" for the Reply-To header only when the reply is to a private message topic.
Corrected and added appropriate specs to confirm correct behaviour.

Tests ensure that the site name is used for public replies and the username is used for private replies.
Merge pull request #2421 from peternlewis/reply_to_name

Use an appropriate name in the Reply-To header