Fix crash that occurred when headers contained non-whitespace runs of a certain length.
Fix crash that occurred when headers contained non-whitespace runs of a certain length.

emit_folded() was trying to slice a string from a to b where a > b because it assumed that
it would have found a space before reaching the line length limit.

niax/rust-email,hoodie/rust-email,buster/rust-email,buster/rust-email,untitaker/rust-email,hoodie/rust-email,niax/rust-email