RetrievalMethod isn't supported?

[nathanhinish]

I have an ID provider that is sending an encrypted SAML response, but it is using the xmldsig tag RetrievalMethod. Currently, xmlenc.js is expecting <KeyInfo><EncryptedKey><EncryptionMethod> and throws an error on line 194.

Here's the GIST for the XML response that causes it: https://gist.github.com/nathanhinish/debe15ddf8e9e134c719