Initial support for preferred content types.

See http://drupal.org/node/475242 for more details.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>

Index: includes/common.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/common.inc,v
retrieving revision 1.913
diff -u -r1.913 common.inc
--- includes/common.inc	27 May 2009 14:33:41 -0000	1.913
+++ includes/common.inc	28 May 2009 18:05:40 -0000
@@ -158,7 +158,8 @@
  * Retrieve output to be displayed in the head tag of the HTML page.
  */
 function drupal_get_html_head() {
-  $output = "<meta http-equiv=\"Content-Type\" content=\"text/html; charset=utf-8\" />\n";
+  global $content_type;
+  $output = "<meta http-equiv=\"Content-Type\" content=\"{$content_type}; charset=utf-8\" />\n";
   return $output . drupal_add_html_head();
 }
 
@@ -3223,8 +3224,29 @@
   return (($skip_anonymous && $user->uid == 0) || ($token == md5(session_id() . $value . variable_get('drupal_private_key', ''))));
 }
 
+function _drupal_init_content_type() {
+  global $content_type;
+
+  /* this should be exposed byt some API so that themes can specify theis 
+   * preferred content types
+   */
+  $preferred_types = array("application/xhtml+xml", "application/xml", "text/xml");
+
+
+  $content_type = "text/html";  /* default */
+  $http_accept = $_SERVER["HTTP_ACCEPT"];
+  foreach ($preferred_types as $type) {
+    if (stristr($http_accept, $type)) {
+      $content_type = $type;
+      break;
+    }
+  }
+
+}
+
 function _drupal_bootstrap_full() {
   static $called;
+  global $content_type;
 
   if ($called) {
     return;
@@ -3245,8 +3267,11 @@
   set_exception_handler('_drupal_exception_handler');
 
   // Emit the correct charset HTTP header.
-  drupal_set_header('Content-Type', 'text/html; charset=utf-8');
   // Detect string handling method
+  _drupal_init_content_type();
+  drupal_set_header('Content-Type', "$content_type; charset=utf-8");
+  drupal_set_header('Vary', 'Accept');
+
   unicode_check();
   // Undo magic quotes
   fix_gpc_magic();