Handle lexing of input such as just "</".

Previously this would cause the lexer to go in an infinite loop in the "text"
state machine.

This fixes #37.

ext/ragel/base_lexer.rl

@@ -292,6 +292,14 @@
     allowed_text   = any* -- terminate_text;
 
     text := |*
+        # Input such as just "</" or "<?". This rule takes precedence over the
+        # rules below, but only if those don't match.
+        terminate_text => {
+            callback("on_text", data, encoding, ts, te);
+
+            fnext main;
+        };
+
         # Text followed by a special tag, such as "foo<!--"
         allowed_text @{ mark = p; } terminate_text => {
             callback("on_text", data, encoding, ts, mark);

spec/oga/xml/lexer/general_spec.rb

@@ -21,5 +21,17 @@ describe Oga::XML::Lexer do
     example 'lex a > as regular text' do
       lex('>').should == [[:T_TEXT, '>', 1]]
     end
+
+    example 'lex </ as regular text' do
+      lex('</').should == [[:T_TEXT, '</', 1]]
+    end
+
+    example 'lex <! as regular text' do
+      lex('<!').should == [[:T_TEXT, '<!', 1]]
+    end
+
+    example 'lex <? as regular text' do
+      lex('<?').should == [[:T_TEXT, '<?', 1]]
+    end
   end
 end