Examines the
SignableSAMLObject for the need to declare non-visible namespaces
before marshalling and signing, and if required, performs the declarations.
If the object does not already have a cached DOM, does have a signature attached,
and the signature contains a
SAMLObjectContentReference with a transform of either
SignatureConstants#TRANSFORM_C14N_EXCL_OMIT_COMMENTSor
SignatureConstants#TRANSFORM_C14N_EXCL_WITH_COMMENTS,
it declares on the object all non-visible namespaces
as determined by
org.opensaml.core.xml.XMLObject.NamespaceManager#getNonVisibleNamespaces().