Updates to Zend\Http\Client overview page

This PR addresses a number of issues with the `Zend\Http\Client` overview page in the docs, notably:

* Each of the code examples have been tested and I've fixed the ones that didn't work. Several of them called `$client->dispatch()` with no parameters, which would have caused a PHP fatal error.
* I've changed the code example in the quick start section so that it actually makes a HTTP request, rather than simply showing how to instantiate and configure the class. I've also split the code into two examples, since it was showing two ways of doing the same thing.
* I've added an example showing how to connect to a SSL URL which mentions `sslcapath` configuration, as without this users will get an error (when using the default adapter).
* I've added some clarity/code surrounding the use of request objects with `Zend\Http\Client`, as I don't think this was explained very well before.  

The one possibly controversial change: I've removed the big list of class methods from the page. I don't think this adds anything for the user, but makes the page unnecessarily long (a particular bugbear of mine with our docs).