- test and fix receiving text/html attachment in multipart/mixed situations

They are now preserved as attachment, instead of diving into parsing-html
  and simplifying.

- adapt mime-debugging

src/dc_receive_imf.rs

@@ -47,12 +47,10 @@ pub fn dc_receive_imf(
         server_uid,
     );
 
-    /*
-    if std::env::var(crate::DCC_MIME_DEBUG).is_ok() {
+    if std::env::var(crate::DCC_MIME_DEBUG).unwrap_or_default() == "2" {
         info!(context, "dc_receive_imf: incoming message mime-body:");
         println!("{}", String::from_utf8_lossy(imf_raw));
     }
-    */
 
     let mime_parser = MimeParser::from_bytes(context, imf_raw);
     let mut mime_parser = if let Err(err) = mime_parser {