support union type from php documentation

[smoench]

resolves #504

This PR adds support for union types from the PHP documentation. They are defined as followed:

<methodsynopsis>
   <type class="union"><type>int</type><type>false</type></type>
   ...
</methodsynopsis>

Before, when a function was defined with this union type notation and it hasn't been corrected by PHPStan (function map), the return has been determined as void. This PR now determines those union type correctly. Several function are affected as you can see in the diff.

[shish]

That looks great, thank you :D