A multi valued Map.
Simple I2P replacement for org.eclipse.jetty.util.MultiMap
so we don't depend on Jetty utils.
Contains only the methods required by MultiPartRequest.
Does not implement Map. Unsynchronized.
This returns the first item or null.
The Jetty version appears to return the item if only one,
or the entire list if more than one.
Only used by MultiPartRequest.contains() which is unused.
contains() would fail with a ClassCastException if we returned a list here,
which is a bug in MultiPartRequest?