rpmbuild: return epoch value after macro expansion

See fedora-copr#2848 (comment)

On @nforro's recommendation, we should return the expanded value.

> I think it would be safer to use spec.rpm_spec.sourceHeader.epoch
> (which also gives you an integer). Or use
> spec.tags(spec.parsed_sections.package).content.epoch.expanded_value.

rpmbuild/copr_rpmbuild/helpers.py

@@ -407,10 +407,18 @@ def __init__(self, path):
 
     def __getattr__(self, name):
         value = getattr(self.tags, name).value
-        if name == "epoch" and value is not None:
-            return int(value)
         return value
 
+    @property
+    def epoch(self):
+        """
+        Evaluated %{epoch} as integer or None
+        """
+        value = self.safe_attr("epoch")
+        if value in [None, ""]:
+            return None
+        return int(value)
+
     @property
     def exclusivearch(self):
         """