Fix canonicalization: should be without comments

Currently used canonicalization method algorithm `http://www.w3.org/2001/10/xml-exc-c14n#` assumes that all comments are removed during canonicalization. But now it's not true. So, if there is comments inside digested node, the digest will be calculated incorrectly. See http://www.w3.org/TR/2002/REC-xml-exc-c14n-20020718/#sec-Use

It's a little fix to Nokogiri's `canonicalize` method invocation. While it's unclear from the nokogiri docs, but third argument should be exactly `nil` to remove comments from canonicalized result, but by default it's `false`. This inconsistence seems to be fixed in upcoming Nokogiri 1.6.2.