BUG: bug in to_msgpack for timezone aware datetime index
BUG: bug in to_msgpack for no freqstr datetime index (GH5506)

TST: msgpack complex test comparisons fixed

API: allow df.to_msgpack() to return a string (no need to specify None for the path_or_buf)
Merge pull request #5507 from jreback/msgpack_bug

BUG: bug in to_msgpack for timezone aware datetime index