Fix argument count in dynamic stores with named arguments

[IfkumRfnl]

Summary

Code counted formatting arguments incorrectly when named arguments were present. It took the internal metadata slot as a formatting argument. Change made the math correct with

 return data_.size() - (named_info_.empty() ? 0 : 1);

i.e. subtracting 1 from the arguments vector count when named arguments are present which makes the counting correct. The corrected count is also used when constructing basic_format_args. Also regression tests were added.