Timestamps don't round-trip through CSV writer/reader

[bjchambers]

Describe the bug

Create a TimestampNanosecondArray and write it to CSV.
Read the resulting CSV back and it will be parsed as UTF-8 not TimestampNanosecondType.
Cast the parsed string to TimestampNanosecondType::DATA_TYPE and the value will be off by the local timezone offset.

I think the solution may be as simple as having the CSV writer append the Z suffix to indicate the timestamp is in GMT time, but I'm not sure.

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Expected behavior
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Additional context
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[novemberkilo]

@alamb I suspect that #832 addresses this bug. I will check // @bjchambers

[novemberkilo]

Here's what I think is going on.

#832 addresses the issue raised here apache/datafusion#958

In essence, it allows for strings indicating local time to be read as timestamps (and not UTF-8)

The round-trip test suggested in this bug confirmed that the value read in is off by the local timezone offset. The same test confirms that round tripping is sound once the writer appends a Z - i.e. writes out the timestamp in UTC.