Fix some invalid assert.throws statements

[igonro]

While writing my tests for #412, I detected some wrong assert.throws statements. I can fix these after I finish the #412 issue (which is almost ready)

These invalid statements look like this:

assert.throws(() => {
  new dfd.NDframe({ data, isSeries: false, columns: [ "A", "A", "C" ] }),
  Error,
  "ColumnIndexError: Column index must contain unique values";
});

and they should be like this:

assert.throws(
  () => new dfd.NDframe({ data, isSeries: false, columns: [ "A", "A", "C" ] }),
  Error,
  "ColumnIndexError: Column index must contain unique values"
);

I checked it and these wrong tests are passing even if the error message does not match. Fortunately, these tests will fail if no error is thrown during in the execution, so at least is guaranteed that an error will be thrown (but it does not check which type of error).

Thanks!