`Element#attribute` fails to find attribute when namespace URL is duplicated

[tompng]

xml = "<root xmlns='url1' xmlns:ns1='url1' xmlns:ns2='url2' xmlns:ns3='url2' a='' ns1:a='' ns1:b='' ns2:c='' ns3:d=''/>"
node = REXML::Document.new(xml).root

node.attribute 'a', 'url1'
# => a='' (ns1:a should match)
node.attribute 'b', 'url1'
# => nil (ns1:b='' should match)
node.attribute 'c', 'url2'
# => ns2:c='' (OK)
node.attribute 'd', 'url2'
# => nil (ns3:d='' should match)

# Ref:
node.attributes.get_attribute_ns '', 'a'     #=> a=''
node.attributes.get_attribute_ns 'url1', 'a' #=> ns1:a=''
node.attributes.get_attribute_ns 'url1', 'b' #=> ns1:b=''
node.attributes.get_attribute_ns 'url2', 'c' #=> ns2:c=''
node.attributes.get_attribute_ns 'url2', 'd' #=> ns3:d=''

Result of node.attribute 'b', 'url1' has changed in #335 as a bugfix (DOM spec violation) of Attributes#get_namespace.
node.attribute 'a', 'url1' case is related to #151.

The easiest way to fix this is to simply use:

def attribute( name, namespace=nil )
  if namespace
    attributes.get_attribute_ns(namespace, name)
  else
    attributes.get_attribute(name)
  end
end

However, it may introduce incompatibility mentioned in #151.
If this method is fixed as above, I think we don't need to deprecate this method (discussed in #151) because it will be a useful method: a superset of attributs.get_attribute_ns and attributs.get_attribute.

[tompng]

Maybe it's possible to only fix the bug and try to leave fallback like this:

def attribute( name, namespace=nil )
  if namespace
    attributes.get_attribute_ns(namespace, name) || unknown_fuzzy_attribute_search_fallback
  else
    attributes.get_attribute(name) || perhaps_another_fallback_here
  end
end