feat: optimize into_bytes buffer and into_json capacity by body len

Based on https://github.com/http-rs/http-types/pull/506/files by @qwerty2501

src/body.rs

@@ -173,7 +173,8 @@ impl Body {
     /// # Ok(()) }) }
     /// ```
     pub async fn into_bytes(mut self) -> crate::Result<Vec<u8>> {
-        let mut buf = Vec::with_capacity(1024);
+        let len = usize::try_from(self.len().unwrap_or(0)).status(StatusCode::PayloadTooLarge)?;
+        let mut buf = Vec::with_capacity(len);
         self.read_to_end(&mut buf).await.status(StatusCode::UnprocessableEntity)?;
         Ok(buf)
     }
@@ -273,9 +274,8 @@ impl Body {
     /// # Ok(()) }) }
     /// ```
     #[cfg(feature = "serde")]
-    pub async fn into_json<T: DeserializeOwned>(mut self) -> crate::Result<T> {
-        let mut buf = Vec::with_capacity(1024);
-        self.read_to_end(&mut buf).await?;
+    pub async fn into_json<T: DeserializeOwned>(self) -> crate::Result<T> {
+        let buf = self.into_bytes().await?;
         serde_json::from_slice(&buf).status(StatusCode::UnprocessableEntity)
     }