added failing test for async transform

when using async transform, if the input data comes faster
than the async process can deal with it, the stringifier
is used instead. This is because line 259 of index.coffee
says to use the stringifier if still tranforming and because
the async transformer has not called the callback yet it is
still transforming. This results in a bunch of empty rows
followed by the header (if used) and a single line of output.

One solution I came up with was altering the mentioned line
to only check if preserve was set, but I don't know enough
to know if that fixes everything or just my use case.