fix: scope variables used by Agent constructor

Some packages that consume http(s).Agent (e.g. tunnel-agent) attempt to construct http.Agent using .constructor and without providing the required constructor parameters.

package.json

@@ -97,6 +97,7 @@
   "scripts": {
     "build": "rm -fr ./dist && NODE_ENV=production babel ./src --out-dir ./dist --copy-files --source-maps && flow-copy-source src dist",
     "create-readme": "gitdown ./.README/README.md --output-file ./README.md",
+    "dev": "NODE_ENV=development babel ./src --out-dir ./dist --copy-files --source-maps --watch",
     "lint": "eslint ./src ./test && flow",
     "test": "NODE_ENV=test nyc ava --verbose --serial"
   },

src/factories/createGlobalProxyAgent.js

@@ -96,25 +96,37 @@ export default (configurationInput: ProxyAgentConfigurationInputType = defaultCo
     return proxyController.HTTP_PROXY;
   };
 
-  const httpAgent = new HttpProxyAgent(
-    isProxyConfigured(getHttpProxy),
-    mustUrlUseProxy(getHttpProxy),
-    getUrlProxy(getHttpProxy),
-    http.globalAgent,
-    eventEmitter
-  );
+  const BoundHttpProxyAgent = class extends HttpProxyAgent {
+    constructor () {
+      super(
+        isProxyConfigured(getHttpProxy),
+        mustUrlUseProxy(getHttpProxy),
+        getUrlProxy(getHttpProxy),
+        http.globalAgent,
+        eventEmitter
+      );
+    }
+  };
+
+  const httpAgent = new BoundHttpProxyAgent();
 
   const getHttpsProxy = () => {
     return proxyController.HTTPS_PROXY || proxyController.HTTP_PROXY;
   };
 
-  const httpsAgent = new HttpsProxyAgent(
-    isProxyConfigured(getHttpsProxy),
-    mustUrlUseProxy(getHttpsProxy),
-    getUrlProxy(getHttpsProxy),
-    https.globalAgent,
-    eventEmitter
-  );
+  const BoundHttpsProxyAgent = class extends HttpsProxyAgent {
+    constructor () {
+      super(
+        isProxyConfigured(getHttpsProxy),
+        mustUrlUseProxy(getHttpsProxy),
+        getUrlProxy(getHttpsProxy),
+        https.globalAgent,
+        eventEmitter
+      );
+    }
+  };
+
+  const httpsAgent = new BoundHttpsProxyAgent();
 
   // Overriding globalAgent was added in v11.7.
   // @see https://nodejs.org/uk/blog/release/v11.7.0/