add form() method

src/error.rs

@@ -6,6 +6,11 @@ use std::fmt;
 pub enum Error {
     /// An HTTP error from the `hyper` crate.
     Http(::hyper::Error),
+    /// An error trying to serialize a value.
+    ///
+    /// This may be serializing a value that is illegal in JSON or
+    /// form-url-encoded bodies.
+    Serialize(Box<StdError>),
     /// A request tried to redirect too many times.
     TooManyRedirects,
     #[doc(hidden)]
@@ -16,6 +21,7 @@ impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {
             Error::Http(ref e) => fmt::Display::fmt(e, f),
+            Error::Serialize(ref e) => fmt::Display::fmt(e, f),
             Error::TooManyRedirects => {
                 f.pad("Too many redirects")
             },
@@ -28,6 +34,7 @@ impl StdError for Error {
     fn description(&self) -> &str {
         match *self {
             Error::Http(ref e) => e.description(),
+            Error::Serialize(ref e) => e.description(),
             Error::TooManyRedirects => "Too many redirects",
             Error::__DontMatchMe => unreachable!()
         }
@@ -36,6 +43,7 @@ impl StdError for Error {
     fn cause(&self) -> Option<&StdError> {
         match *self {
             Error::Http(ref e) => Some(e),
+            Error::Serialize(ref e) => Some(&**e),
             Error::TooManyRedirects => None,
             Error::__DontMatchMe => unreachable!()
         }
@@ -54,7 +62,11 @@ impl From<::url::ParseError> for Error {
     }
 }
 
-
+impl From<::serde_urlencoded::ser::Error> for Error {
+    fn from(err: ::serde_urlencoded::ser::Error) -> Error {
+        Error::Serialize(Box::new(err))
+    }
+}
 
 /// A `Result` alias where the `Err` case is `reqwest::Error`.
 pub type Result<T> = ::std::result::Result<T, Error>;