PHP requires boolean typehints to be written "bool" and disallows
"boolean" as an alias. This changes the error messages to match
the actual type name and avoids confusing messages like "must be
of type boolean, boolean given".

This a followup to ce1d69a1f6dcf15d43029301059c25e5bc09a577, which
implements the same change for integer->int.

We don't want ReflectionMethod::invoke() to simply ignore its first argument,
if the method to invoke is a static method. Instead we match its ZPP with
that of ReflectionMethod::invokeArgs(). Furthermore, we apply the DRY
principle by factoring out the code to a common helper function to prevent
inadvertent future divergence of the implementations of both methods.

As can be seen from the necessity to adapt some test cases, this causes a
BC break for some pathological cases. Therefore we apply this patch to PHP
7.1 only, which is still in beta phase.