GHSA-f67m-9j94-qv9j: Parser creates invalid uninitialized value

Affected versions of this crate called mem::uninitialized() in the HTTP1 parser to create values of type httparse::Header (from the httparse crate). This is unsound, since Header contains references and thus must be non-null.

The flaw was corrected by avoiding the use of mem::uninitialized(), using MaybeUninit instead.