Use xml-exc-c14n Canonicalizer by default

[Stummi]

The SAML Specification states the following

5.4.3 Canonicalization Method
SAML implementations SHOULD use Exclusive Canonicalization [Excl-C14N], with or without comments,
both in the <ds:CanonicalizationMethod> element of <ds:SignedInfo>, and as a
<ds:Transform> algorithm. Use of Exclusive Canonicalization ensures that signatures created over
SAML messages embedded in an XML context can be verified independent of that context.

right now, the Canonicalization used by default through the dsig library is another one ( C14N11).

Using the C14N11 does not work with every IDP, since some only support the recommendet Excl-C14N standard.

While the Canonicalization can be chosen through a param passed to the SAMLServiceProvider, it would be sensible to follow the spec by default, if no option is provided