Comment_save should not overwrite hostname if already set

Problem/Motivation

This stems from the following issue: #1334830: Missing comment date
The feeds_comment_processor module uses the comment_save function from the drupal api comment.module. There are cases in which the $comment->hostname is passed into the comment_save function, but currently the hostname on a new comment is always reset to the ip_address().

Proposed resolution

Similar to the other comment fields, the $comment->hostname should be checked if it is set before setting a default with ip_address(). I will attach a proposed patch.

API changes

comment_save will not overwrite the hostname value if it is already set.