Bugfix - subject is an array

With the following XML from Okta:

```xml
<?xml version="1.0" encoding="UTF-8"?>
<saml2:Assertion
	xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion" ID="id17359508616939371931038386" IssueInstant="2014-12-08T20:21:46.345Z" Version="2.0">
	<saml2:Issuer Format="urn:oasis:names:tc:SAML:2.0:nameid-format:entity">http://www.okta.com/Issuer</saml2:Issuer>
	<saml2:Subject>
		<saml2:NameID Format="urn:oasis:names:tc:SAML:1.1:nameid-format:unspecified">userName</saml2:NameID>
		<saml2:SubjectConfirmation Method="urn:oasis:names:tc:SAML:2.0:cm:bearer">
			<saml2:SubjectConfirmationData NotOnOrAfter="2014-12-08T20:26:46.345Z" Recipient="https://cabinet.d2ldev.com/api/saml2"/>
		</saml2:SubjectConfirmation>
	</saml2:Subject>
	<saml2:Conditions NotBefore="2014-12-08T20:16:46.345Z" NotOnOrAfter="2014-12-08T20:26:46.345Z">
		<saml2:AudienceRestriction>
			<saml2:Audience>https://cabinet.d2ldev.com</saml2:Audience>
		</saml2:AudienceRestriction>
	</saml2:Conditions>
	<saml2:AuthnStatement AuthnInstant="2014-12-08T20:21:46.345Z">
		<saml2:AuthnContext>
			<saml2:AuthnContextClassRef>urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport</saml2:AuthnContextClassRef>
		</saml2:AuthnContext>
	</saml2:AuthnStatement>
</saml2:Assertion>
```

Subject is an array when it enters the `parse` function in `saml20.js`. This fix populates the `nameIdentifierClaimType` correctly.