The position should be counted for each nodeset but the previous
implementation counts position for union-ed nodeset.

For example, "/a/*/*[1]" should be matched to "<c1/>" and "<c2/>" with
the following XML.

    <a>
      <b>
        <c1/>
      </b>
      <b>
        <c2/>
      </b>
    </a>

But the previous implementation just returns only "<c1/>".

* lib/rexml/element.rb (REXML::Attributes#each_attribute):
  Support Enumerator for no block use.

* lib/rexml/element.rb (REXML::Attributes#each):
  Support Enumerator for no block use.

* lib/rexml/functions.rb (REXML::Functions.string):
  Support NaN again.

* lib/rexml/xpath_parser.rb: Re-implement "Step" evaluator.
  It should evaluate "AxisSpecifier", "NodeTest" and "Predicate" in one
  step to respect position for each nodeset.

* test/rexml/test_jaxen.rb: Enable more tests. Remained tests should
  be also enabled but it'll not be near future.

* test/rexml/xpath/test_base.rb: Fix expected value.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63237 b2dd03c8-39d4-4d8f-98ff-823fe69b080e