RNTester http_server send cookie fix

Summary:
Signed-off-by: Evan J Brunner <ej3@appitto.me>

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Motivation can be found in #17899

This `RNTester/js/http_test_server.js` is part of a internal websocket test suite / devtool.

Can be tested with `curl -D - localhost:5556` observing that the `Set-Cookie: wstest=OK; Path=\` header is present, and the service throws no exceptions.. etc

[INTERNAL][MINOR][./RNTester/js/http_test_server.js] - fixed set cookie with connect framework
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  CATEGORY
[----------]        TYPE
[ CLI      ]   [-------------]      LOCATION
[ DOCS     ]   [ BREAKING    ]   [-------------]
[ GENERAL  ]   [ BUGFIX      ]   [-{Component}-]
[ INTERNAL ]   [ ENHANCEMENT ]   [ {File}      ]
[ IOS      ]   [ FEATURE     ]   [ {Directory} ]   |-----------|
[ ANDROID  ]   [ MINOR       ]   [ {Framework} ] - | {Message} |
[----------]   [-------------]   [-------------]   |-----------|

[CATEGORY] [TYPE] [LOCATION] - MESSAGE

 EXAMPLES:

 [IOS] [BREAKING] [FlatList] - Change a thing that breaks other things
 [ANDROID] [BUGFIX] [TextInput] - Did a thing to TextInput
 [CLI] [FEATURE] [local-cli/info/info.js] - CLI easier to do things with
 [DOCS] [BUGFIX] [GettingStarted.md] - Accidentally a thing/word
 [GENERAL] [ENHANCEMENT] [Yoga] - Added new yoga thing/position
 [INTERNAL] [FEATURE] [./scripts] - Added thing to script that nobody will see
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Closes https://github.com/facebook/react-native/pull/17900

Differential Revision: D6977087

Pulled By: hramos

fbshipit-source-id: af6205343fccf69c57e0c26a85a5b04d61288a23

RNTester/js/http_test_server.js

@@ -30,11 +30,7 @@ const app = connect();
 
 app.use(function(req, res) {
   console.log('received request');
-  const cookieOptions = {
-    //httpOnly: true, // the cookie is not accessible by the user (javascript,...)
-    secure: false, // allow HTTP
-  };
-  res.cookie('wstest', 'OK', cookieOptions);
+  res.setHeader('Set-Cookie', ['wstest=OK; Path=/']);
   res.end('Cookie has been set!\n');
 });