* MM-15741: Omit Reply-To email header if EmailSettings.ReplyToAddress is blank. * MM-15741: Adds tests. * MM-15741: Changes interface name because the '-er' convention doesn't apply. * MM-15741: Combines two structs and un-exports. * MM-15741: Switches to table test. * MM-15741: Renames test. * MM-15741: Unexport interface. * MM-15741: Change to allow test cacheing.