Test types (#4140)

* Distinguish request and response data types

* Fix Axios headers type

`axios.headers` is not of the same type as `request.headers`, so a new type
`AxiosDefaults` was introduced

* Replace grunt-ts with dtslint

This asserts that the type definitions are valid in the specified TypeScript
version and above. This is the same tool that is used by DefinitelyTyped.

* Remove grunt-ts

* Restore typescript dependency

* Fix missing semicolons

Co-authored-by: Claas Augner <github@caugner.de>
Co-authored-by: Jay <jasonsaayman@gmail.com>

package.json

@@ -5,7 +5,7 @@
   "main": "index.js",
   "types": "index.d.ts",
   "scripts": {
-    "test": "grunt test",
+    "test": "grunt test && dtslint",
     "start": "node ./sandbox/server.js",
     "build": "NODE_ENV=production grunt build",
     "preversion": "grunt version && npm test",
@@ -35,6 +35,7 @@
   "devDependencies": {
     "abortcontroller-polyfill": "^1.5.0",
     "coveralls": "^3.0.0",
+    "dtslint": "^4.1.6",
     "es6-promise": "^4.2.4",
     "grunt": "^1.3.0",
     "grunt-banner": "^0.6.0",
@@ -44,7 +45,6 @@
     "grunt-eslint": "^23.0.0",
     "grunt-karma": "^4.0.0",
     "grunt-mocha-test": "^0.13.3",
-    "grunt-ts": "^6.0.0-beta.19",
     "grunt-webpack": "^4.0.2",
     "istanbul-instrumenter-loader": "^1.0.0",
     "jasmine-core": "^2.4.1",