Always suppress "Expect: 100-continue" header

cURL automatically adds an "Expect: 100-continue" header to all POST/PUT requests larger than 1 KB ([source code](https://github.com/bagder/curl/blob/576ac00eb396dd7fb2d12d0fe4b6fdeeba5071e6/lib/http.c#L2617)). It then waits for up to 1 second for a "100 Continue" response before sending the POST/PUT body ([source code](https://github.com/bagder/curl/blob/56120ca04bc6c0dc4e942504c7abf018b7ef8428/lib/transfer.c#L1973)).

The purpose of this waiting is to give the server a chance to send e.g. a 403 Forbidden, before the client sends a big payload over the wire. This concern is not relevant when talking to a WebDriver server.

The WebDriver implementation embedded in PhantomJS does not support Expect, so my Behat tests spend a lot of time waiting for these 1 second timeouts. Suppressing the Expect header makes them run about 2-3x faster (using PHP 5.3.10).

php-webdriver already suppresses the Expect header in some circumstances (see PR #41). With this PR it is always suppressed.
> The comment says, that it will force 1 second delay for server, which doesn't understand Expect header. Is that wise?

The 1 second delay is caused by cURL's default behaviour. I suppose it is wise - at least, it is intentional. Adding an empty Expect header suppresses this behaviour. The behaviour is still compliant with the HTTP spec.

> Any specific HTTP server list that doesn't support that header, e.g. Apache/Nginx/etc.?

HTTP/1.1-compliant servers MUST support Expect. But simple and/or special-purpose web servers like that in PhantomJS often only support HTTP/1.0.