safehasattr: pass attribute name as string instead of bytes

This is a step toward replacing `util.safehasattr` usage with plain `hasattr`.
The builtin function behave poorly in Python2 but this was fixed in Python3.

These change are done one by one as they tend to have a small odd to trigger
puzzling breackage.

mercurial/hgweb/server.py

@@ -100,7 +100,7 b' class _httprequesthandler(httpservermod.'
 
     def log_request(self, code='-', size='-'):
         xheaders = []
-        if util.safehasattr(self, b'headers'):
+        if util.safehasattr(self, 'headers'):
             xheaders = [
                 h for h in self.headers.items() if h[0].startswith('x-')
             ]