Fix a bug with string keys in coverage

Fix a problem with different keys in coverage info.

In !64 (merged) I replaced coverage data transfer from file to memory structure. And total coverage is sometimes cached in a file. json.load and json.dump leads to replace integer keys to string keys. When files were everywhere, it works. When some data is transferred via memory and some data is read from a file, there is mixing keys.

Fix it by a workaround: read from file and cast.