Add more header validation (#188)

This adds validation of header values on receive, and of both header
names and header values on send. This doesn't change the return
type of set to be a Result, it just validates when the request is
sent. Also removes the section in the README describing handling
of invalid headers, and updates a test that verified acceptance of
non-ASCII headers so that it verifies rejection of them instead.

src/lib.rs

@@ -7,7 +7,6 @@
 //! * Minimal dependency tree
 //! * Obvious API
 //! * Blocking API
-//! * Convenience over correctness
 //! * No use of unsafe
 //!
 //! ```