Fix validate jwt essential logic

ref: #445

Author: lepture

authlib/jose/rfc7519/claims.py

@@ -53,7 +53,7 @@ def __getattr__(self, key):
 
     def _validate_essential_claims(self):
         for k in self.options:
-            if self.options[k].get('essential') and k not in self:
+            if self.options[k].get('essential') and not self.get(k):
                 raise MissingClaimError(k)
 
     def _validate_claim_value(self, claim_name):