fix: initialize _format in CSVReader constructor

[bigerl]

I had trouble with the following code snippet:

csv::CSVFormat format;
format.delimiter(',').quote(false).no_header();
csv::CSVReader csv_reader(csv_file, format);
for (csv::CSVRow& row: csv_reader) { 
   // do sth with the rows 
}

Bug: First line is still parsed as headers not as a row like reported in #132 .

Cause: field _format was not initialized. That caused the function call in the following line to rely on the default constructed value of field CSVFormat _format:

csv-parser/include/internal/csv_reader.cpp

Line 169 in 621a9d9

this->initial_read();

Fix: Initialize the field _format in the constructor CSVReader::CSVReader(csv::string_view filename, CSVFormat format).
In the other constructors it is actually initialized. So it was probably simply looked over.

[vincentlaucsb]

Thanks for catching this.