Exclude possibly sensitive data from logs

Header fields and data frames can potentially contain sensitive data.
This change omits these from Debug output which reduces the chance that
this ends up in logs.

src/frame/data.rs

@@ -119,6 +119,7 @@ impl<T> fmt::Debug for Data<T> {
             .field("stream_id", &self.stream_id)
             .field("flags", &self.flags)
             .field("pad_len", &self.pad_len)
+            // `data` purposefully excluded
             .finish()
     }
 }