Fix Json extractor to be 32kB by default

While technically the default limit for `JsonBody` is indeed 256kB when
called through `JsonBody::new`, in practice the limit is always set to
32kB for every context where `JsonBody` is used (via `JsonConfig`).
This is also consistent with the documentation of the
`JsonError::Overflow`, which mentions a default maximum of `32kB`.

This commit changes the default to also be `32kB` instead of `256kB`.

src/types/json.rs

@@ -345,15 +345,15 @@ where
         let payload = payload.take();
 
         JsonBody::Body {
-            limit: 262_144,
+            limit: 32_768,
             length,
             payload,
             buf: BytesMut::with_capacity(8192),
             _res: PhantomData,
         }
     }
 
-    /// Set maximum accepted payload size. The default limit is 256kB.
+    /// Set maximum accepted payload size. The default limit is 32kB.
     pub fn limit(self, limit: usize) -> Self {
         match self {
             JsonBody::Body {