Set group the same as owner (file resource)

[ryotarai]

If owner is set and group is not set, set group the same value as owner automatically.

[yamajun]

If set "same value" as group in default, cause error with user belongs to not same name group.

I recommend use "run_specinfra(:get_user_gid, ...)" for retrieve user's GID like this pull-request.
nownabe/itamae-plugin-resource-authorized_keys#4