Fix prefix argument for read_csv/read_table
Closes #5732 

can you add some tests for this?
 One question I had about this - while use of the `prefix` argument now matches the documentation, this change is no longer backward compatible (if a user passed an arbitrary string and expected the columns to be named X1, X2,...)
 @nmichaud prob just put a note in v0.14.txt to note that
 OK, added a test for the prefix argument.
 @nmichaud actually just add a release notes entry in API for 0.13...and i'll throw it in...thanks
 @jreback, I'll add the release notes, you vetted this?
 Will push release note directly.
