Support milliseconds in cacheDuration parsing

Since the introduction of cacheDuration parsing in 25cbddd we've been
seeing parsing failures for one of our IdPs, whose cacheDuration value
is set to

    cacheDuration="PT6H0M0.000S"

This seems like a perfectly valid ISO8601 duration - the regexp being
used for parsing doesn't provide for the possibility of milliseconds.

This adds an optional non-captured group to the regexp to permit them.
Since the captured string value is already being converted using .to_f,
it appears this should work as-is.

To achieve minimal test coverage I've added a seconds string to one of
the existing metadata examples with a zero value; this reproduces the
failure and confirms the fix without needing to add a specific test
case.

Author: Simon Coffey

lib/onelogin/ruby-saml/utils.rb

@@ -15,7 +15,7 @@ class Utils
 
       DSIG      = "http://www.w3.org/2000/09/xmldsig#"
       XENC      = "http://www.w3.org/2001/04/xmlenc#"
-      DURATION_FORMAT = %r(^(-?)P(?:(?:(?:(\d+)Y)?(?:(\d+)M)?(?:(\d+)D)?(?:T(?:(\d+)H)?(?:(\d+)M)?(?:(\d+)S)?)?)|(?:(\d+)W))$)
+      DURATION_FORMAT = %r(^(-?)P(?:(?:(?:(\d+)Y)?(?:(\d+)M)?(?:(\d+)D)?(?:T(?:(\d+)H)?(?:(\d+)M)?(?:(\d+(?:\.\d{3})?)S)?)?)|(?:(\d+)W))$)
 
       # Checks if the x509 cert provided is expired
       #

test/metadata/idp_descriptor_6.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
-<md:EntityDescriptor entityID="https://hello.example.com/access/saml/idp.xml" validUntil="2020-01-04T18:02:33.910Z" cacheDuration="P2DT" xmlns:md="urn:oasis:names:tc:SAML:2.0:metadata">
+<md:EntityDescriptor entityID="https://hello.example.com/access/saml/idp.xml" validUntil="2020-01-04T18:02:33.910Z" cacheDuration="P2DT0.000S" xmlns:md="urn:oasis:names:tc:SAML:2.0:metadata">
   <md:IDPSSODescriptor WantAuthnRequestsSigned="true" protocolSupportEnumeration="urn:oasis:names:tc:SAML:2.0:protocol">
     <md:KeyDescriptor use="signing">
       <ds:KeyInfo xmlns:ds="http://www.w3.org/2000/09/xmldsig#">