Student cannot create Wiki if force-format is ON (in separate/visible group mode)

Wiki settings:

• mode: collaborative
• force format: checked
• group mode: Separate groups | Visible groups

The instructor has created the first page of the wiki (so a wiki_subwiki record is created with groupid=0 and userid=0 for the wiki).

Students will see "You can not view/edit this page"

The cause:
mod/wiki/create.php calls $wikipage->create_page() in two situations:
#1. User submitted a form where they selected the format
#2. Force-format is checked

The function page_wiki_create::create_page() has been modified recently (commit 595b68e2fdb22351972 for MDL-30478). It now ignores $this->gid used to be in there (set by create.php in situation #2), and only consider the $data->groupinfo from form-submission (situation #1). Thus, when force-format is checked (#2), the $groupid becomes '0', the instructor's subwiki record is retrieved instead (groupid=0). This record will prevent the student from editing (as groupid 0 won't match the student's groupid)

So, the code can be modified from:
================
if (isset($data->groupinfo)) {
$groupid = $data->groupinfo;
} else {
$groupid = '0';
}
===============

to something like this (verified that it fixes the problem on my end):
================
if (isset($data->groupinfo)) {
$groupid = $data->groupinfo;
} else if (!empty($this->gid)) {
$groupid = $this->gid;
} else {
$groupid = '0';
}
===============

I'm not sure if this would affect/break anything else.