SAML metadata Content-Type should be application/samlmetadata+xml

**Expected Behavior**
The IANA-registered MIME type for SAML metadata is `application/samlmetadata+xml`.

**Current Behavior**
The `Saml2MetadataFilter` responds with `Content-Type: application/xml`.

**Context**
Not a big deal for human users looking to save and/or copy-paste the data, but automated tools may care, and standards should be followed where possible.