fix warnings in tests

[alisterburt]

• pandas will require pyarrow in the future
• delim_whitespace is being removed in pd.read_csv

[alisterburt]

hey @jojoelfe - you're right this is the PR that's giving me trouble.

Summary:

• there's a deprecation in pandas, they're removing delim_whitespace in favour of sep/delimiter``=\s+
• the behaviour isn't equivalent to delim_whitespace for some of our empty string tests and breaks them

[jojoelfe]

Hey, just pushed a PR against the pyarrow branch. I think the problem was somewhere else. Before I was resetting columns that contain only NaNs to their original string, but that ment setting a column of type float to a string. Now the whole columns are being replaced, which also replaces the dtype.

[alisterburt]

all green, let's goooo