Gateway MVC ServerRequest body broken when created from another

[mcordeiro73]

Describe the bug
When using ServerRequest.from(other) to augment a ServerRequest in a filter, a new DefaultServerRequestBuilder is returned with body = new byte[0], even if the original ServerRequest has a body. Subsequent filters then get an empty body value. It does appear that the downstream service still does get the correct body, and only other filters are affected.

Sample

Below is a very trivial sample before filter that reproduces the issue. Note: the request body does not need to be read from the original ServerRequest to reproduce this issue, I'm merely doing that in the example to illustrate the issue.

builder.before(request -> {
	ServerRequest modified;
	try {
		String requestBody = request.body(String.class);
		modified = ServerRequest.from(request)
				.header("X-CUSTOM_HEADER", "value")
				.build();
		String modifiedRequestBody = request.body(String.class);
		Assert.isTrue(StringUtils.equals(requestBody, modifiedRequestBody), "Request body should still be the same!");
	} catch (Exception e) {
		throw new RuntimeException(e);
	}
	return modified;
});