http::Body: add bytes_contents, like contents but gives Bytes (#132)

Its trivial for the library to provide this efficient implementation,
but building one out of the `contents` method will be inefficient and
cost a copy.

Since the Body constructors includes From<Bytes>, this makes a
corresponding accessor.

Author: pchickey

src/http/body.rs

@@ -47,6 +47,9 @@ pub mod util {
 /// * `async fn Body::contents(&mut self) -> Result<&[u8], Error>` is ready
 ///   when all contents of the body have been collected, and gives them as a
 ///   byte slice.
+/// * `async fn Body::bytes_contents(&mut self) -> Result<Bytes, Error>` is
+///   the same as `Body::contents`, except returns the `Bytes` type instead of
+///   a byte slice.
 /// * `async fn Body::str_contents(&mut self) -> Result<&str, Error>` is ready
 ///   when all contents of the body have been collected, and gives them as a str
 ///   slice.
@@ -170,6 +173,16 @@ impl Body {
             }
         }
     }
+    /// Collect the entire contents of this `Body`, and expose it as `Bytes`.
+    /// This async fn will be pending until the entire `Body` is
+    /// copied into memory, or an error occurs.
+    pub async fn bytes_contents(&mut self) -> Result<Bytes, Error> {
+        let _ = self.contents().await?;
+        match &self.0 {
+            BodyInner::Complete { data, .. } => Ok(data.clone()),
+            _ => unreachable!(),
+        }
+    }
 
     /// Get a value for the length of this `Body`'s content, in bytes, if
     /// known. This value can come from either the Content-Length header