Clean up RequestParts API (#167)

In http-body 0.4.3 `BoxBody` implements `Default`. This allows us to
clean up the API of `RequestParts` quite a bit.

src/extract/rejection.rs

@@ -13,14 +13,15 @@ use tower::BoxError;
 define_rejection! {
     #[status = INTERNAL_SERVER_ERROR]
     #[body = "Extensions taken by other extractor"]
-    /// Rejection used if the method has been taken by another extractor.
+    /// Rejection used if the request extension has been taken by another
+    /// extractor.
     pub struct ExtensionsAlreadyExtracted;
 }
 
 define_rejection! {
     #[status = INTERNAL_SERVER_ERROR]
     #[body = "Headers taken by other extractor"]
-    /// Rejection used if the URI has been taken by another extractor.
+    /// Rejection used if the headers has been taken by another extractor.
     pub struct HeadersAlreadyExtracted;
 }
 
@@ -94,13 +95,6 @@ define_rejection! {
     pub struct BodyAlreadyExtracted;
 }
 
-define_rejection! {
-    #[status = INTERNAL_SERVER_ERROR]
-    #[body = "Cannot have two `Request<_>` extractors for a single handler"]
-    /// Rejection type used if you try and extract the request more than once.
-    pub struct RequestAlreadyExtracted;
-}
-
 define_rejection! {
     #[status = BAD_REQUEST]
     #[body = "Form requests must have `Content-Type: x-www-form-urlencoded`"]
@@ -272,6 +266,19 @@ composite_rejection! {
     }
 }
 
+composite_rejection! {
+    /// Rejection used for [`Request<_>`].
+    ///
+    /// Contains one variant for each way the [`Request<_>`] extractor can fail.
+    ///
+    /// [`Request<_>`]: http::Request
+    pub enum RequestAlreadyExtracted {
+        BodyAlreadyExtracted,
+        HeadersAlreadyExtracted,
+        ExtensionsAlreadyExtracted,
+    }
+}
+
 /// Rejection used for [`ContentLengthLimit`](super::ContentLengthLimit).
 ///
 /// Contains one variant for each way the