make prior class not get overwritten

I noticed that the prior class for the gw priors isn't being saved properly in result files.

This is because we only keep the classes of conditional prior dicts. This means that the prior in CBC results won't respect constraints.

bilby/core/result.py

@@ -299,7 +299,9 @@ class Result(object):
     @priors.setter
     def priors(self, priors):
         if isinstance(priors, dict):
-            if not isinstance(priors, PriorDict):
+            if isinstance(priors, PriorDict):
+                self._priors = priors
+            else:
                 self._priors = PriorDict(priors)
             if self.parameter_labels is None:
                 self.parameter_labels = [self.priors[k].latex_label for k in