* add request.body
* Update koa-bodyparser.d.ts
According to the documentation of `koa-bodyparser`:
```javascript
// the parsed body will store in this.request.body
// if nothing was parsed, body will be an empty object {}
ctx.body = ctx.request.body;
```
Therefore the `body` property will exit in the `request` object.
Changing the `body` property from optional to required can also fix the conflict problem in Koa's declaration file:
```ts
class Koa extend Request, Response {}
```
Without this, there will be `Module '"koa-bodyparser"' resolves to a non-module entity and cannot be imported using this construct. (2497)` error
This solution is found (here)[https://github.com/Microsoft/TypeScript/issues/5073].
koa-json's `.d.ts` file has such namespace.