Let v4 support w3c drivers

lib/protocol/init.js

@@ -56,10 +56,40 @@ export default function init (desiredCapabilities = {}) {
         }
     })
 
-    return this.requestHandler.create({
-        path: '/session',
-        method: 'POST'
-    }, {
-        desiredCapabilities: this.desiredCapabilities
-    })
+    const jsonwpCaps = this.desiredCapabilities;
+    const w3cCapKeys = [
+        "browserName",
+        "browserVersion",
+        "platformName",
+        "acceptInsecureCerts",
+        "pageLoadStrategy",
+        "proxy",
+        "setWindowRect",
+        "timeouts",
+        "strictFileInteractability",
+        "unhandledPromptBehavior",
+        "goog:chromeOptions",
+        "moz:firefoxOptions"
+    ];
+    const w3cCapsAlwaysMatch = {};
+    w3cCapKeys.forEach(key => {
+        if (jsonwpCaps[key]) {
+            w3cCapsAlwaysMatch[key] = jsonwpCaps[key];
+        }
+    });
+    const w3cCaps = {
+        alwaysMatch: w3cCapsAlwaysMatch,
+        firstMatch: [{}]
+    };
+
+    return this.requestHandler.create(
+        {
+            path: "/session",
+            method: "POST"
+        },
+        {
+            desiredCapabilities: jsonwpCaps,
+            capabilities: w3cCaps
+        }
+    );
 }