Support string-length() XPath function without an argument

[SAY-5]

Per XPath 1.0, string-length() defaults to the string-value of the context node when called without an argument, but Functions.string_length required a positional argument and raised ArgumentError.

REXML::XPath.match(REXML::Document.new('<root><item>123</item><item>1234</item><item>12345</item></root>'), '//item[string-length()=4]')
# before: wrong number of arguments (given 0, expected 1) (ArgumentError)
# after:  matches the <item>1234</item> element

Defaults the argument to @context[:node] to match string and normalize_space.