Fix failing tests for 0.10.x, 4.x and 5.x

Travis build failed on "Uncaught TypeError: Object.keys called on non-object".
That's a real error (res._headers was initially null until a header is set).

lib/cors-anywhere.js

@@ -414,7 +414,7 @@ exports.createServer = function createServer(options) {
 
     // When the error occurs after setting headers but before writing the response,
     // then any previously set headers must be removed.
-    var headerNames = res.getHeaderNames ? res.getHeaderNames() : Object.keys(res._headers);
+    var headerNames = res.getHeaderNames ? res.getHeaderNames() : Object.keys(res._headers || {});
     headerNames.forEach(function(name) {
       res.removeHeader(name);
     });

test/test.js

@@ -392,6 +392,16 @@ describe('Proxy errors', function() {
   });
 
   it('Invalid HTTP status code', function(done) {
+    // Strict HTTP status validation was introduced in Node 4.5.5+, 5.11.0+.
+    // https://github.com/nodejs/node/pull/6291
+    var nodev = process.versions.node.split('.').map(function(v) { return parseInt(v); });
+    if (nodev[0] < 4 ||
+        nodev[0] === 4 && nodev[1] < 5 ||
+        nodev[0] === 4 && nodev[1] === 5 && nodev[2] < 5 ||
+        nodev[0] === 5 && nodev[1] < 11) {
+      this.skip();
+    }
+
     var errorMessage = 'RangeError [ERR_HTTP_INVALID_STATUS_CODE]: Invalid status code: 0';
     if (parseInt(process.versions.node, 10) < 9) {
       errorMessage = 'RangeError: Invalid status code: 0';