fix(headers): don't use Show to write UserAgent header

Signed-off-by: Peter Atashian <retep998@gmail.com>

src/header/common/user_agent.rs

@@ -1,5 +1,5 @@
 use header::{Header, HeaderFormat};
-use std::fmt::{self, Show};
+use std::fmt;
 use header::shared::util::from_one_raw_str;
 
 /// The `User-Agent` header field.
@@ -22,10 +22,16 @@ impl Header for UserAgent {
 
 impl HeaderFormat for UserAgent {
     fn fmt_header(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        let UserAgent(ref value) = *self;
-        value.fmt(fmt)
+        fmt.write_str(&*self.0)
     }
 }
 
 bench_header!(bench, UserAgent, { vec![b"cargo bench".to_vec()] });
 
+#[test] fn test_format() {
+    use std::borrow::ToOwned;
+    use header::Headers;
+    let mut head = Headers::new();
+    head.set(UserAgent("Bunnies".to_owned()));
+    assert!(head.to_string() == "User-Agent: Bunnies\r\n".to_owned());
+}