handle infinity like nan

infinity and nan are not part of json standard. the default python
json serialiser will dump `inf` and `nan` (unquoted) and load the
them.

we've historically returned `nan` and supported ignoring it by specifying
`fill=dropna`.

i don't believe `"inf"` or `"nan"` is significantly better and since
i don't want to break existing behaviour of nan, i think it's best
to just treat infinity as we do nan for now. anything more opinionated
probably requires more discussion.

related: #1023