<th> elements in data rows are ignored

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An example which I was working on; https://en.wikipedia.org/wiki/Template:2019%E2%80%9320_coronavirus_pandemic_data/Russia_medical_cases

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The non-backwards-compatible way to fix this is s/"td"/"td,th"/ on https://github.com/mk12/table-extract/blob/master/src/lib.rs#L161.