StatusCode::as_str should return a &'static str (try 3)

[dekellum]

After various other changes, make StatusCode::as_str return a &'static str.

Since 'static is a longer lifetime, I don't think this constitutes a breaking change, but like #443, this could be merged for release of a new MINOR (0.3.0) or MAJOR (1.0.0), just to be cautious.

(Supersedes) closes: #380, closes: #436 (I rebased #380 here).