Allow empty date setting to dateBone

Setting a date back to empty is not possible using fromClient right now: Empty string is converted to None, and None is not allowed by super-call to baseBone.isInvalid() as valid value. So this might fix the problem and integrates with [#32](viur-framework/viur-vi#32) in vi.

bones/dateBone.py

@@ -99,7 +99,7 @@ def fromClient( self,valuesCache, name, data ):
 		"""
 		rawValue = data.get(name, None)
 		if not rawValue:
-			value = None
+			value = rawValue
 		elif unicode(rawValue).replace("-",  "",  1).replace(".","",1).isdigit():
 			if int(rawValue) < -1*(2**30) or int(rawValue)>(2**31)-2:
 				value = False  # its invalid
@@ -158,7 +158,7 @@ def fromClient( self,valuesCache, name, data ):
 		else:
 			err = self.isInvalid(value)
 			if not err:
-				valuesCache[name] = value
+				valuesCache[name] = value if value else None
 				if value is None:
 					return "No value entered"
 			return err