XPath.match, XPath.first, XPath.each, XPathParser#parse and XPathParser#match accepted nodeset as element.
This pull request changes the first parameter of these method to be an element instead of nodeset.
Passing nodeset will be deprecated.

# Documented usage. OK
REXML::XPath.match(element, xpath)

# Undocumented usage. Deprecate in this pull request
nodeset = [element]
REXML::XPath.match(nodeset, xpath)

Background

#249 will introduce a temporary cache.

def parse path, nodeset
  path_stack = @parser.parse( path )
  nodeset.first.document.send(:enable_cache) do
    match( path_stack, nodeset )
  end
end

But the signature XPathParser#match(path, nodeset) does not guarantee that all nodes in the nodeset has the same root document.
So cache does not work in the code below. It's still slow.

REXML::XPath.match(2.times.map { REXML::Document.new('<a>'*400+'</a>'*400) }, 'a//a')

The interface is holding our back, so I propose to drop accepting array as element.
This change is a backward incompatibility, but it just drops undocumented feature. I think only the test code was unintentionally using this feature.

XPath.match with array

XPath.match only traverse the first element of the array for some selectors.

nodeset = [REXML::Document.new("<a><b/></a>"), REXML::Document.new("<a><c/></a>")]

REXML::XPath.match(nodeset, "a/*")
#=> [<b/>, <c/>]

REXML::XPath.match(nodeset, "//a/*")
#=> [<b/>] # I expect [<b/>, <c/>] but the second document is ignored

It indicates that XPath.match is not designed to search inside multiple nodes/documents.