fix computation of test-values when distribution parameters are given by a theano TensorConstant (#1554)

Author: burnpanck

pymc3/distributions/distribution.py

@@ -71,6 +71,9 @@ def getattr_value(self, val):
         if isinstance(val, tt.TensorVariable):
             return val.tag.test_value
 
+        if isinstance(val, tt.TensorConstant):
+            return val.value
+
         return val