gem: Fix get_installed_versions: correctly parse "default" version. (#783) (#918)

* Fix get_installed_versions: correctly parse "default" version.

gem query output of

    bundler (default: 2.1.4, 1.17.2)

Gets parsed as:

    ['default:', '1.17.2']

Fix this by skipping "default: " if present in the list of versions - by adding
it as an optional part of the regex, grouped as a non-capturing group to keep
the index of existing group.

This now correctly parses the above input as

    ['2.1.4:', '1.17.2']

Fixes #782

* Fix gem get_installed_versions (cont): add changelog fragment

* Update changelogs/fragments/783-fix-gem-installed-versions.yaml as per suggestion

Co-authored-by: Felix Fontein <felix@fontein.de>

Co-authored-by: Felix Fontein <felix@fontein.de>
(cherry picked from commit 6dc98c08fd)

Co-authored-by: Vlad Mencl <vladimir.mencl@reannz.co.nz>

changelogs/fragments/783-fix-gem-installed-versions.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+  - "gem - fix get_installed_versions: correctly parse ``default`` version (https://github.com/ansible-collections/community.general/pull/783)."

plugins/modules/packaging/language/gem.py

@@ -157,7 +157,7 @@ def get_installed_versions(module, remote=False):
     (rc, out, err) = module.run_command(cmd, environ_update=environ, check_rc=True)
     installed_versions = []
     for line in out.splitlines():
-        match = re.match(r"\S+\s+\((.+)\)", line)
+        match = re.match(r"\S+\s+\((?:default: )?(.+)\)", line)
         if match:
             versions = match.group(1)
             for version in versions.split(', '):