[FEATURE] Add a warning when parsing the same header twice for one row

[lkappeler]

Lost data and debugged the parser. It turned out I had the same header name twice inside my CSV
I noticed some difference in the input and output of the parsed file, after some investigation I figured out some fields like description, usually containing html with line breaks where empty. Looked in the parser code, as I expected some errors with the line breaks inside the parsed value. How ever the parser works great and I found no Issues with parsing the content properly. It turned out that I used the same header multiple times ( from joining some tables to generate the CSV), therefore the first value was overwritten with the second one which is usually empty.

Describe the solution you'd like
A warning would be nice when a already set property on the row gets overwritten. Probably somewhere here. Further more a option could be introduced to rename the header if he already exits.

[doug-martin]

I changed fast-csv to emit an error when duplicate headers are encountered. If you would like to transform them to prevent duplicates headers now accepts a function to transform them.