Don't send requests with Connection:keep-alive if we have no agent.

Avoids unhandleable ECONNRESETs.

I think this is arguably a Node http bug (in 0.10.18 at least), but: if you run

```js
http.request({headers: {connection: 'keep-alive'}, agent: false});
```

During the `ClientRequest` constructor, `self.shouldKeepAlive` is set to false because there is no agent. But then it calls (indirectly) the `storeHeader` function (in `http.js`, which sets `self.shouldKeepAlive` to true because you specified the keep-alive header.

Then once the request is over `responseOnEnd` (in `http.js`) runs.  Because `shouldKeepAlive` is true, we do NOT destroy the underlying socket. But we do remove its error listener.  However, because we do NOT have an agent, nobody cares that we emit `free`, and the socket is essentially leaked.

That is, we continue to have an open socket to the target server, which has no `error` listener and which will never be cleaned up.

It's bad enough that this is a resource leak. But to make matters worse: let's say that the target server dies.  This socket will emit an ECONNRESET error... and since there is no `error` listener on the socket (there's a listener on the `ClientRequest`! but not on the socket), bam, time for an incredibly confusing error to be thrown from the top level, probably crashing your process or triggering `uncaughtException`.

I think the best workaround here is to ensure that if we have no agent, then we don't send connection: keep-alive. This PR is one implementation of this.

(I'll work around this a different way in Meteor for now: by providing an explicit Agent.)