Fix DigestMethod lookup bug.

# Thanks!

Thanks for maintaining the ruby-saml library. This is a great service you're providing. It's been a while since I found a bug in ruby-saml. Here's a fix for this one. I hope you find the tests satisfactory.

# Passes validation

&lt;Signature **xmlns:ds**='http://www.w3.org/2000/09/xmldsig#'&gt;
...
&lt;**ds:DigestMethod** Algorithm='http://www.w3.org/2001/04/xmlenc#sha256'/&gt;

# Fails validation with a Digest Mismatch error

&lt;Signature **xmlns**='http://www.w3.org/2000/09/xmldsig#'&gt;
...
&lt;**DigestMethod** Algorithm='http://www.w3.org/2001/04/xmlenc#sha256'/&gt;

# Details

This bug is masked by sha1 being the default `digest_algorithm` when the lookup of the `DigestMethod` fails.