快应用下请求头的设置的bug

**问题描述**
调用`uni.request`这个api的时候，设置header的为`{'Content-Type': 'application/x-www-form-urlencoded'}`,实际请求后content-type为`application/json`，导致所有请求体在`requeset payload`里


结果：


**系统信息:**
所有安卓平台

Start by reproducing the issue with uni.request on an Android quick app, using the Content-Type header shown in the report. Compare the actual request header and payload with the requested form-encoded content type; done means the supplied header is preserved and the request body is sent accordingly.

Written by the indexing model from the issue text.