fix: properly read namespaces with no prefix from the root element

[Unix4ever]

Another attempt to fix: https://github.com/crewjam/saml/pull/580

The code is reading the attributes from the elements in the loop, then writes to a map using childEl.Scope as the key. Intention behind this code is not clear, as the resulting map contains a random string for an empty space, depending on which element was the last one.

Instead, write the key to the map only if the childEl is the element we're encoding.

Tested that against Fusion Auth SAML and Microsoft SP with SAML. Can't submit the ACS/Metadata for Microsoft for the testing purposes, as it contains the sensitive data.