diff --git a/lib/Request.js b/lib/Request.js
index 03f18eb2d6f..f8797a9dcba 100644
--- a/lib/Request.js
+++ b/lib/Request.js
@@ -71,10 +71,13 @@ class Request {
     }
 
     /**
-     * @param {!Object} headers
+     * @param {string} key
+     * @param {string} value
      */
-    setHeaders(headers) {
-        this._headersOverride = headers;
+    setHeader(key, value) {
+        if (!this._headersOverride)
+            this._headersOverride = Object.assign({}, this._headers);
+        this._headersOverride[key] = value;
     }
 
     /**
diff --git a/third_party/phantomjs/test/module/webpage/add-header.js b/third_party/phantomjs/test/module/webpage/add-header.js
index 5594f4e2834..46b70bb1b0e 100644
--- a/third_party/phantomjs/test/module/webpage/add-header.js
+++ b/third_party/phantomjs/test/module/webpage/add-header.js
@@ -1,4 +1,3 @@
-//! unsupported
 async_test(function () {
     var webpage = require('webpage');