update Error to implement Send

BREAKING CHANGE: This does change the type of `Error::Serialize`, but
  should only possibly affect those constructing `Error`, not consuming it.

src/error.rs

@@ -10,7 +10,7 @@ pub enum Error {
     ///
     /// This may be serializing a value that is illegal in JSON or
     /// form-url-encoded bodies.
-    Serialize(Box<StdError>),
+    Serialize(Box<StdError + Send + Sync>),
     /// A request tried to redirect too many times.
     TooManyRedirects,
     #[doc(hidden)]
@@ -50,6 +50,12 @@ impl StdError for Error {
     }
 }
 
+fn _assert_types() {
+    fn _assert_send<T: Send>() {
+    }
+    _assert_send::<Error>();
+}
+
 impl From<::hyper::Error> for Error {
     fn from(err: ::hyper::Error) -> Error {
         Error::Http(err)