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/request.rs

@@ -112,6 +112,9 @@ impl Request {
     }
 
     fn do_call(&self, payload: Payload) -> Result<Response> {
+        for h in &self.headers {
+            h.validate()?;
+        }
         let response = self.to_url().and_then(|url| {
             let reader = payload.into_read();
             let unit = Unit::new(&self, &url, true, &reader);