RUST-1698 Fix reading gridfs chunks from async-std file stream (#950)

Author: abr-egn

src/gridfs/upload.rs

@@ -231,6 +231,9 @@ where
             n => n,
         };
         total_bytes_read += bytes_read;
+        if total_bytes_read == buf.len() {
+            break;
+        }
     }
 
     Ok(total_bytes_read)