Implicitly convert DataObjectHandleBase to str properties

This helps the transition to data handles whenever we have the pattern
`B.Input = A.Output` and `A.Output` is changed to a data handle and `B.Input`
is still a string.

GaudiKernel/python/GaudiKernel/PropertyProxy.py

@@ -44,6 +44,10 @@ def _isCompatible(tp, value):
         if (type(value) is str) or derives_from(value, 'Configurable'):
             # we can set string properties only from strings or configurables
             return value
+        elif isinstance(value, DataObjectHandleBase):
+            # Implicitly convert DataObjectHandleBase to str for backward
+            # compatiblity in cases like B.Input (str) = A.Output (handle)
+            return str(value)
         else:
             raise ValueError(errmsg)
     elif (tp in [list, tuple, dict]):

GaudiKernel/tests/nose/test_Configurables.py

@@ -34,6 +34,12 @@ def test_correct():
         'DataHandle': DataObjectHandleBase('/Event/X')}
 
 
+def test_str_from_datahandle():
+    a = MyAlg()
+    a.Text = DataObjectHandleBase('value')
+    assert a.getProp('Text') == 'value'
+
+
 def test_invalid_value():
     a = MyAlg()
     try: