Fix error return from HMAC signing method (#371)

Author: ss49919201

hmac.go

@@ -91,7 +91,7 @@ func (m *SigningMethodHMAC) Verify(signingString string, sig []byte, key interfa
 func (m*SigningMethodHMAC) Sign(signingStringstring, keyinterface{}) ([]byte, error) {
 ifkeyBytes, ok:=key.([]byte); ok {
 if!m.Hash.Available() {
-returnnil, newError("HMAC sign expects []byte", ErrInvalidKeyType)
+returnnil, ErrHashUnavailable
  }
 
 hasher:=hmac.New(m.Hash.New, keyBytes)
@@ -100,5 +100,5 @@ func (m *SigningMethodHMAC) Sign(signingString string, key interface{}) ([]byte,
 returnhasher.Sum(nil), nil
  }
 
-returnnil, ErrInvalidKeyType
+returnnil, newError("HMAC sign expects []byte", ErrInvalidKeyType)
 }