Fix XPath queries using default XML namespace

This fixes https://gitlab.com/yorickpeterse/oga/issues/195

lib/oga/xpath/compiler.rb

@@ -1391,14 +1391,19 @@ module Oga
         if ns and ns != STAR
           if @namespaces
             ns_uri = @namespaces[ns]
-            ns_match = 
+            ns_match =
               if ns_uri
                 input.namespace.and(input.namespace.uri.eq(string(ns_uri)))
               else
                 self.false
               end
           else
-            ns_match = input.namespace_name.eq(string(ns))
+            ns_match =
+              if ns == XML::Element::XMLNS_PREFIX
+                input
+              else
+                input.namespace_name.eq(string(ns))
+              end
           end
 
           condition = condition ? condition.and(ns_match) : ns_match

spec/oga/xpath/compiler/paths_spec.rb

@@ -68,4 +68,26 @@ describe Oga::XPath::Compiler do
       end
     end
   end
+
+  describe 'querying elements with a default namespace' do
+    before do
+      @document = parse('<a xmlns="n" xmlns:ns1="x">Foo<b></b><b></b><ns1:c></ns1:c></a>')
+
+      @a1 = @document.children[0]
+      @b1 = @a1.children[1]
+      @b2 = @a1.children[2]
+    end
+
+    describe '/xmlns:a' do
+      it 'returns a NodeSet' do
+        expect(evaluate_xpath(@document)).to eq(node_set(@a1))
+      end
+    end
+
+    describe '//xmlns:b' do
+      it 'returns a NodeSet' do
+        expect(evaluate_xpath(@document)).to eq(node_set(@b1, @b2))
+      end
+    end
+  end
 end