ENH: read_excel: try converting numeric to int

All Excel numeric data is stored as floats, so have to convert
specifically. Changing default because it's suprising to save something
with what looks like a row/column of integers and get a column of floats
instead. (especially because it can lead to annoying Float64Indexes)
TST: Test for float --> int in the middle of object dtype
Merge pull request #5394 from jtratner/fix-int-handling-excel

ENH: read_excel: try converting numeric to int