Inf is not handled correctly. How should it be handled?

[TimG1964]

Inf is a Float64 and XLSX.jl treats it as such. When writing to an xlsx file, Inf is written in a cell like this:

<c r="C5" t="n" s="15">
<v>Inf</v>
</c>

Excel therefore expects a number (t="n") but it gets a string. This generates an Excel error when the file is opened. Excel can fix it by turning the cell type to a string anyway.

It is not obvious to me how Inf should be written to an XLSX file, but pre-emptively turning it into a string may be satisfactory.

Edit: I think this is a different manifestation of the same problem that is causing #179. Both NaN and Inf are of type Float64.