Default fetch body option to undefined. Resolves #138.

- `null` values cause issues with Edge's fetch implementation
- TODO: add test cases verifying fetch args, and consider browser testing to catch issues in vendor-specific implementations

src/middleware.js

@@ -133,7 +133,7 @@ function apiMiddleware({ getState }) {
         var res = await fetch(endpoint, {
           ...options,
           method,
-          body,
+          body: body || undefined,
           credentials,
           headers: headers || {}
         });