Allow keeping bool return values

PyMI contains a workaround to avoid returning boolean values for
void MI functions, mostly in an attempt to remain compatible with
other WMI libraries.

The problem is that we're losing boolean return values. We could
just drop the workaround and let PyMI return boolean values even
for void functions, however that might break some applications.

Instead, we'll add an optional argument called
"keep_bool_ret_vals", which can explicitly request the boolean
return values to be preserved.

wmi/__init__.py

@@ -681,6 +681,7 @@ def invoke_method(self, target, method_name, *args, **kwargs):
         params = self._get_method_params(target, method_name)
         operation_options = self._get_mi_operation_options(
             operation_options=kwargs.pop('operation_options', None))
+        keep_bool_ret_vals = kwargs.pop('keep_bool_ret_vals', False)
 
         for i, v in enumerate(args):
             _, el_type, _ = params.get_element(i)
@@ -710,7 +711,8 @@ def invoke_method(self, target, method_name, *args, **kwargs):
                 # returns void, as there's no direct way to determine it.
                 # This won't work if the method is expected to return a
                 # boolean value!!
-                if element != ('ReturnValue', mi.MI_BOOLEAN, True):
+                if (element != ('ReturnValue', mi.MI_BOOLEAN, True)
+                        or keep_bool_ret_vals):
                     l.append(self._wrap_element(*element))
             return tuple(l)