Read the full file without line translation

It seems like the length reported by the channel and the one that
`really_read` expects do not match when some line ending translation is
happening. Thus try to avoid the translation.

Author: Leonidas-from-XIV

test/test_write.ml

@@ -21,7 +21,7 @@ let to_file_tests =
     let output_file = Filename.temp_file "test_yojson_to_file" ".json" in
     Yojson.Safe.to_file ?suf output_file Fixtures.json_value;
     let file_content =
-      let ic = open_in output_file in
+      let ic = open_in_bin output_file in
       let length = in_channel_length ic in
       let s = really_input_string ic length in
       close_in ic;