Auto merge of #6171 - eleweek:fix-4184-ok, r=Manishearth

I tried fixing #4184 , here is the code I have right now. 

I haven't tested it, because I don't know what is the best way to test this part of code. Would like some help with testing this. Should I write an autotest or should I just test manually?

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components/net/resource_task.rs

@@ -99,8 +99,23 @@ pub fn start_sending_sniffed_opt(start_chan: LoadConsumer, mut metadata: Metadat
                                  -> Result<ProgressSender, ()> {
     if opts::get().sniff_mime_types {
         // TODO: should be calculated in the resource loader, from pull requeset #4094
-        let nosniff = false;
-        let check_for_apache_bug = false;
+        let mut nosniff = false;
+        let mut check_for_apache_bug = false;
+
+        if let Some(ref headers) = metadata.headers {
+            if let Some(ref raw_content_type) = headers.get_raw("content-type") {
+                if raw_content_type.len() > 0 {
+                    let ref last_raw_content_type = raw_content_type[raw_content_type.len() - 1];
+                    check_for_apache_bug = last_raw_content_type == b"text/plain"
+                                        || last_raw_content_type == b"text/plain; charset=ISO-8859-1"
+                                        || last_raw_content_type == b"text/plain; charset=iso-8859-1"
+                                        || last_raw_content_type == b"text/plain; charset=UTF-8";
+                }
+            }
+            if let Some(ref raw_content_type_options) = headers.get_raw("X-content-type-options") {
+                nosniff = raw_content_type_options.iter().any(|ref opt| *opt == b"nosniff");
+            }
+        }
 
         let supplied_type = metadata.content_type.map(|ContentType(Mime(toplevel, sublevel, _))| {
             (format!("{}", toplevel), format!("{}", sublevel))