Fix two bugs in MakeAbsolute filter involving base URIs that have empty path.

The bugs are:
* Undefined $is_folder variable when path is empty, and
* Improper concatenation of host and path together.

Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>

tests/HTMLPurifier/URIFilter/MakeAbsoluteTest.php

@@ -114,6 +114,13 @@ class HTMLPurifier_URIFilter_MakeAbsoluteTest extends HTMLPurifier_URIFilterHarn
         $this->assertFiltering('javascript: window.location = \'http://www.example.com\';', false);
     }
     
+    // miscellaneous
+    
+    function testFilterDomainWithNoSlash() {
+        $this->setBase('http://example.com');
+        $this->assertFiltering('foo', 'http://example.com/foo');
+    }
+    
     // error case
     
     function testErrorNoBase() {