Support direct combinators in CSS :has()

Adds support for CSS queries like "a:has(> b)", "a:has(~ b)", and
"a:has(+ b)".

Closes sparklemotion#688.

lib/nokogiri/css/xpath_visitor.rb

@@ -51,7 +51,8 @@ def visit_function node
         when /^comment\(/
           "comment()"
         when /^has\(/
-          ".//#{node.value[1].accept(self)}"
+          is_direct = node.value[1].value[0].nil? # e.g. "has(> a)", "has(~ a)", "has(+ a)"
+          ".#{"//" if !is_direct}#{node.value[1].accept(self)}"
         else
           args = ['.'] + node.value[1..-1]
           "#{node.value.first}#{args.join(', ')})"

test/css/test_parser.rb

@@ -65,6 +65,9 @@ def test_to_a
       def test_has
         assert_xpath  "//a[.//b]", @parser.parse("a:has(b)")
         assert_xpath  "//a[.//b/c]", @parser.parse("a:has(b > c)")
+        assert_xpath  "//a[./b]", @parser.parse("a:has(> b)")
+        assert_xpath  "//a[./following-sibling::b]", @parser.parse("a:has(~ b)")
+        assert_xpath  "//a[./following-sibling::*[1]/self::b]", @parser.parse("a:has(+ b)")
       end
 
       def test_dashmatch