Update csv.hpp

[alongL]

for centos 7.6 , gcc-4.8 will produce some compile error for these things.

1. "" _csv， there must be a space between "" and _csv
2. constexpr to const. if not compile error occours

csv.hpp:3915:29: error：enclosing class of constexpr non-static member function ‘bool csv::CSVRow::iterator::operator==(const csv::CSVRow::iterator&) const’ is not a literal type
             constexpr  bool operator==(const iterator& other) const {