Remove Request#setConnectionKeepAlive()

Oracle JDK defaults [`http.keepAlive`](https://docs.oracle.com/javase/8/docs/technotes/guides/net/http-keepalive.html) to `true`. Scribe implicitly **modifies this behaviour globally at runtime** by setting the system property to `false` (by default) in [`Request.java#createConnection()`](https://github.com/fernandezpablo85/scribe-java/blob/master/src/main/java/org/scribe/model/Request.java#L84) to fix [some unspecified pre-Gingerbread connection problem](https://github.com/fernandezpablo85/scribe-java/commit/145c009fc3734958d18625286f31e43fc19e8734).

In my oppinion scribe model must not tamper with JDK defaults and hence `connectionKeepAlive` property has to be removed.