Uses header name key based array for header assignation in Guzzle 4 requests

In Guzzle 4, the `$request->addHeaders($headers)` method expects to receive an associative array indexed by header names.

The headers array can contains scalar values or sub-arrays.

This PR fixes the `WebApiContext` context to be Guzzle 4 compatible.

Note: The tests passed before since the assertion is based only on the presence of the header name/value in the response body, but the sent headers where not formatted correctly.

This is the previous output from the test application embed with the extension for testing purpose:
````json
{
    "warning":"Do not expose this service in production : it is intrinsically unsafe",
    "method":"GET",
    "headers": {
        "host":["localhost:8080"],
        "user-agent":["Guzzle\/4.0.2 curl\/7.21.4 PHP\/5.5.6"],
        "0":["foobar:bazquux"]
    }
}
````

After:
````json
{
    "warning":"Do not expose this service in production : it is intrinsically unsafe",
    "method":"GET",
    "headers": {
        "host":["localhost:8080"],
        "user-agent":["Guzzle\/4.0.2 curl\/7.21.4 PHP\/5.5.6"],
        "foobar":["bazquux"]
    }
}
````