Bump koa from 2.7.0 to 2.8.1

Bumps [koa](https://github.com/koajs/koa) from 2.7.0 to 2.8.1.
- [Release notes](https://github.com/koajs/koa/releases)
- [Changelog](https://github.com/koajs/koa/blob/master/History.md)
- [Commits](koajs/koa@2.7.0...2.8.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

package.json

@@ -16,7 +16,7 @@
   ],
   "devDependencies": {
     "coveralls": "3.0.4",
-    "koa": "2.7.0",
+    "koa": "2.8.1",
     "koa-mount": "4.0.0",
     "mocha": "6.1.4",
     "mocha-lcov-reporter": "1.3.0",

yarn.lock

@@ -1058,10 +1058,10 @@ koa-mount@4.0.0:
     debug "^4.0.1"
     koa-compose "^4.1.0"
 
-koa@2.7.0:
-  version "2.7.0"
-  resolved "https://registry.yarnpkg.com/koa/-/koa-2.7.0.tgz#7e00843506942b9d82c6cc33749f657c6e5e7adf"
-  integrity sha512-7ojD05s2Q+hFudF8tDLZ1CpCdVZw8JQELWSkcfG9bdtoTDzMmkRF6BQBU7JzIzCCOY3xd3tftiy/loHBUYaY2Q==
+koa@2.8.1:
+  version "2.8.1"
+  resolved "https://registry.yarnpkg.com/koa/-/koa-2.8.1.tgz#98e13b267ab8a1868f015a4b41b5a52e31457ce5"
+  integrity sha512-iCgjRkntcVud5amnioi+79r7TPVhKPfCY8UzL9GjyU+nIWTE9xsqyrJ6kib3InT9JYRyUu9Y67i5apCjf997TQ==
   dependencies:
     accepts "^1.3.5"
     cache-content-type "^1.0.0"