docs: notes contentType's value (#349)

Author: changzhiwin

README.md

@@ -125,7 +125,7 @@ httpclient.request('http://nodejs.org', function (err, body) {
     - ***writeStream*** [stream.Writable](http://nodejs.org/api/stream.html#stream_class_stream_writable) - A writable stream to be piped by the response stream. Responding data will be write to this stream and `callback` will be called with `data` set `null` after finished writing.
     - ***files*** {Array<ReadStream|Buffer|String> | Object | ReadStream | Buffer | String - The files will send with `multipart/form-data` format, base on `formstream`. If `method` not set, will use `POST` method by default.
     - ***consumeWriteStream*** [true] - consume the writeStream, invoke the callback after writeStream close.
-    - ***contentType*** String - Type of request data. Could be `json`. If it's `json`, will auto set `Content-Type: application/json` header.
+    - ***contentType*** String - Type of request data. Could be `json` (**Notes**: not use `application/json` here). If it's `json`, will auto set `Content-Type: application/json` header.
     - ***nestedQuerystring*** Boolean - urllib default use querystring to stringify form data which don't support nested object, will use [qs](https://github.com/ljharb/qs) instead of querystring to support nested object by set this option to true.
     - ***dataType*** String - Type of response data. Could be `text` or `json`. If it's `text`, the `callback`ed `data` would be a String. If it's `json`, the `data` of callback would be a parsed JSON Object and will auto set `Accept: application/json` header. Default `callback`ed `data` would be a `Buffer`.
     - **fixJSONCtlChars** Boolean - Fix the control characters (U+0000 through U+001F) before JSON parse response. Default is `false`.