Fix generation of non-optional object parameters that can be null.

[digilist]

Consider the following example:

class Foo {
    public function doSomething(\stdClass $a = null, $b) {
    }
}

By generating a method from the ReflectionMethod of the given code above, the generated code did not consider the default value from $a. This has the effect, that one cannot pass null into that variable any more.

This PR fixes this. See also Ocramius/ProxyManager#337

[adamlundrigan]

Makes sense to me but I'll defer to @Ocramius