Response: Don't forget chunked_threshold

Make sure any set `chunked_threshold` carries over when boxing or replacing
data.

src/response.rs

@@ -264,7 +264,7 @@ impl<R> Response<R> where R: Read {
             headers: self.headers,
             status_code: self.status_code,
             data_length: data_length,
-            chunked_threshold: None,
+            chunked_threshold: self.chunked_threshold,
         }
     }
 
@@ -392,7 +392,7 @@ impl<R> Response<R> where R: Read + Send + 'static {
             status_code: self.status_code,
             headers: self.headers,
             data_length: self.data_length,
-            chunked_threshold: None,
+            chunked_threshold: self.chunked_threshold,
         }
     }
 }