BUG/ENH: Fix to_excel representation of inf values

Fixes #6782 by converting inf values to string (default is `'inf'`).
This still round trips successfully. According to Stack Overflow post I
found, there isn't a specific representation of `INF` values -
http://stackoverflow.com/questions/15704538/how-can-i-express-nan-values-in-excel
so we're going to make the decision *for* the writers. This also
(implicitly) fixes an issues where round-tripping an Excel file with an
inf value would cause an `OverflowError` and fail to read back.


cc @jmcnamara and @arthurgerigk-rocket
Looks like *-inf* is being written for positive infinite values and vice versa.