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.
Then comment short say: `Removes 1 second delay caused by non HTTP/1.1 compliant servers`.

Since it's curl who is adding that header, not the code that composes the request, then we can run into situation when 1st request is 20KB and all 20KB is received instantly. This completely defies usages of `Except` header.