okhttp Interceptor 中为post等body添加额外字段（MultipartRequestBody 可能有问题，未测试）

gistfile1.txt

client.interceptors().add(new Interceptor() {
            @Override
            public Response intercept(Chain chain) throws IOException {
                final Request request = chain.request();

                if (!HttpMethod.requiresRequestBody(request.method())) {
                    return chain.proceed(request);
                }

                final RequestBody newRb = new FormEncodingBuilder()
                        .add("testExtra", "123")
                        .build();
                final RequestBody oldBody = request.body();
                Request nowReq = request.newBuilder().method(request.method(), new RequestBody() {
                    @Override
                    public long contentLength() throws IOException {
                        if (oldBody.contentLength() <= 0) {
                            return newRb.contentLength();
                        }
                        return oldBody.contentLength() + 1 + newRb.contentLength();
                    }

                    @Override
                    public MediaType contentType() {
                        return request.body().contentType();
                    }

                    @Override
                    public void writeTo(BufferedSink sink) throws IOException {
                        if (oldBody.contentLength() > 0) {
                            request.body().writeTo(sink);
                            sink.writeByte('&');
                        }
                        newRb.writeTo(sink);
                    }
                }).build();

                return chain.proceed(nowReq);
            }
        });