Add support for loadBeforeEx

zc.zlibstorage wraps base storage and includes generic __getattr__
to forward all unknown attribute access to base:

https://github.com/zopefoundation/zc.zlibstorage/blob/6d5a3c75/src/zc/zlibstorage/__init__.py#L52-L53

But if base implements loadBeforeEx (zopefoundation/ZODB#323)
the following scenario is then possible:

ZODB sees that zlibstorage provides loadBeforeEx and loads data via
loadBeforeEx instead of loadBefore, but the data are loaded
not decompressed, and ZODB complains about "broken pickle".

-> let's fix this by explicitly wrapping loadBeforeEx if base storage
provides it.

src/zc/zlibstorage/__init__.py

@@ -40,6 +40,9 @@ def __init__(self, base, *args, **kw):
             self._transform = lambda data: data
         self._untransform = decompress
 
+        if hasattr(base, 'loadBeforeEx'):
+            self.loadBeforeEx = self._loadBeforeEx
+
         for name in self.copied_methods:
             v = getattr(base, name, None)
             if v is not None:
@@ -59,6 +62,12 @@ def load(self, oid, version=''):
         data, serial = self.base.load(oid, version)
         return self._untransform(data), serial
 
+    def _loadBeforeEx(self, oid, tid):
+        data, serial = self.base.loadBeforeEx(oid, tid)
+        if data is not None:
+            data = self._untransform(data)
+        return data, serial
+
     def loadBefore(self, oid, tid):
         r = self.base.loadBefore(oid, tid)
         if r is not None:
@@ -151,7 +160,7 @@ class ServerZlibStorage(ZlibStorage):
     """
 
     copied_methods = ZlibStorage.copied_methods + (
-        'load', 'loadBefore', 'loadSerial', 'store', 'restore',
+        'load', 'loadBefore', 'loadBeforeEx', 'loadSerial', 'store', 'restore',
         'iterator', 'storeBlob', 'restoreBlob', 'record_iternext',
         )