fix: excel data type handling

[talagluck]

update test .xlsx file
add excel tests to deal with columns with multiple data types

Closes #3052

[tychoish]

@talagluck, so I think my changes to your SLT are really just a rendering case: (e.g. strings aren't output with quotes, and sometimes floats without decimals are rendered as their integer components, and space column separators are always used.)

I'm pretty sure we get the empty string/null value handling correct, based on the code, but some more poking might be useful.

[talagluck]

@tychoish that's all totally fine. The main thing I wanted to confirm is that the HEADING didn't turn into NULL when the column header argument was false. The tests as they are now look great.

[tychoish]

@scsmithr probably ready for review here!