Allow setting User-Agent

[asbjornu]

To be able to distinguish different implementaions of this library from one another, it should be possible to set a $userAgent parameter that is used in a User-Agent header which is sent every HTTP request against PayEx' API.

It should be possible to set custom HTTP headers in the stream_context key of the options array parameter of the SoapClient constructor:

// We should of course figure out which version this library as well as PHP we're running dynamically and not hard coded liket his:
$userAgent = $this->_userAgent . 'PayEx.Ecommerce.Php/1.0.2 php/7.0.6';
// The stream_context needs to include the 'ssl' key if 'ssl_verify' is set to true.
$context = stream_context_create(array('http' => array('header' => "User-Agent: $userAgent"));
$options = array('stream_context' => $context);
$client = new \SoapClient($wsdl, $options);

Since the stream_context is already initialized in __construct(), the header key can just be added there along with the already existing ssl key being set if ssl_verify is true.

The format of the User-Agent header should be as per RFC 7231 section 5.5.3. We should include as much information in the header as possible, such as the version of PHP and any library relevant to the HTTP request. Simply put, it should look something like this:

User-Agent: PayEx.Magento/1.2.3 PayEx.Ecommerce.Php/1.0.2 Magento/1.9.2.4 PHP/7.0.6

[aait]

Solved in 3042b84