Fix prefix argument for read_csv/read_table

[nmichaud]

Closes #5732

[jreback]

can you add some tests for this?

[nmichaud]

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,...)

[jreback]

@nmichaud prob just put a note in v0.14.txt to note that

[nmichaud]

OK, added a test for the prefix argument.

[jreback]

@nmichaud actually just add a release notes entry in API for 0.13...and i'll throw it in...thanks

[ghost]

@jreback, I'll add the release notes, you vetted this?

[ghost]

Will push release note directly.