Change std::fmt::{Float,LowerExp,UpperExp} to not print '-NaN' for f32::NAN and f64::NAN
auto merge of #16425 : nham/rust/fix_nan_format, r=alexcrichton

Currently, this:

    println!("{}", std::f64::NAN);

prints "-NaN". This commit is an attempt to change that to "NaN" instead.