http: use env proxy when agent has no proxyEnv option

When a user creates a custom agent without specifying proxyEnv but
Node.js is configured to use a proxy from the environment (via
--use-env-proxy or NODE_USE_ENV_PROXY), fall back to process.env.

A developer can still explicitly disable proxying for an agent even
when Node.js is configured to use a proxy at runtime, by passing a
falsy proxyEnv explicitly. For example:

    const agent = new https.Agent({ proxyEnv: null });

Signed-off-by: swigger <swigger@gmail.com>

Author: swigger

lib/_http_agent.js

@@ -25,6 +25,7 @@ const {
   NumberIsFinite,
   NumberParseInt,
   ObjectKeys,
+  ObjectPrototypeHasOwnProperty,
   ObjectSetPrototypeOf,
   ObjectValues,
   Symbol,
@@ -184,7 +185,9 @@ function Agent(options) {
       this.options.agentKeepAliveTimeoutBuffer :
       1000;
 
-  const proxyEnv = this.options.proxyEnv;
+  const proxyEnv = ObjectPrototypeHasOwnProperty(this.options, 'proxyEnv') ?
+    this.options.proxyEnv :
+    (getOptionValue('--use-env-proxy') ? process.env : undefined);
   if (typeof proxyEnv === 'object' && proxyEnv !== null) {
     this[kProxyConfig] = parseProxyConfigFromEnv(proxyEnv, this.protocol, this.keepAlive);
     debug(`new ${this.protocol} agent with proxy config`, this[kProxyConfig]);