(#7274) Output 4-digit file modes in File type

When updating file mode, output 4 digit file mode instead of omitting
the leading 0, i.e. 0755 instead of 755. This fully represents the
file mode, and lessens the likelihood of someone incorrectly setting
the mode on a file by copy/pasting the incomplete mode specification.

Signed-off-by: Nan Liu <nan@puppetlabs.com>
Signed-off-by: Jacob Helwig <jacob@puppetlabs.com>