docs: add request.origin

docs/api/context.md

@@ -148,6 +148,7 @@ koa uses [http-assert](https://github.com/jshttp/http-assert) for assertions.
   - `ctx.url`
   - `ctx.url=`
   - `ctx.originalUrl`
+  - `ctx.origin`
   - `ctx.href`
   - `ctx.path`
   - `ctx.path=`

docs/api/request.md

@@ -39,6 +39,15 @@
 
   Get request original URL.
 
+### request.origin
+
+  Get origin of URL, include `protocol` and `host`.
+
+```js
+this.request.origin
+// => http://example.com
+```
+
 ### request.href
 
   Get full request URL, include `protocol`, `host` and `url`.