chore(deps): update dependency koa to v2.6.1

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change	References
koa	devDependencies	minor	2.5.3 -> 2.6.1	source

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Release Notes

koajs/koa

### [`v2.6.1`](https://renovatebot.com/gh/koajs/koa/blob/master/History.md#​261--2018-10-23) [Compare Source](https://renovatebot.com/gh/koajs/koa/compare/2.6.0...2.6.1) ================== **fixes** - \[[`4964242`](http://github.com/koajs/koa/commit/49642428342e5f291eb9d690802e83ed830623b5)] - fix: use X-Forwarded-Host first on app.proxy present ([#​1263](https://renovatebot.com/gh/koajs/koa/issues/1263)) (fengmk2 >) ### [`v2.6.0`](https://renovatebot.com/gh/koajs/koa/blob/master/History.md#​260--2018-10-23) [Compare Source](https://renovatebot.com/gh/koajs/koa/compare/2.5.3...2.6.0) ================== **features** - \[[`9c5c58b`](http://github.com/koajs/koa/commit/9c5c58b18363494976185e7ddc790ac63de840ed)] - feat: use :authority header of http2 requests as host ([#​1262](https://renovatebot.com/gh/koajs/koa/issues/1262)) (Martin Michaelis >) - \[[`9146024`](http://github.com/koajs/koa/commit/9146024e1094e8bb871ab15d1b7fc556a710732f)] - feat: response.attachment append a parameter: options from contentDisposition ([#​1240](https://renovatebot.com/gh/koajs/koa/issues/1240)) (小雷 >) **others** - \[[`d32623b`](http://github.com/koajs/koa/commit/d32623baa7a6273d47be67d587ad4ea0ecffc5de)] - docs: Update error-handling.md ([#​1239](https://renovatebot.com/gh/koajs/koa/issues/1239)) (urugator >)

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