Warn users if headers or fields have "hidden" spaces in them

[waxlamp]

This can lead to a subtle error. If your CSV file's header line has spaces after the commas:

_key, _from, _to

then Arango will not recognize _from and _to as the proper edge table headers. Since this is generally not what a user intends, we should warn about cases like this.

Similar argument for spaces in the actual data rows.