Issue with clearing cache

[alexpchin]

Hello, I've just implemented this library and I believe there is an issue with clearing the internal cache after logging out. I've added a force clear cache to be able to be cleared on Logout:

diff --git a/node_modules/apollo-link-jwt/lib/index.js b/node_modules/apollo-link-jwt/lib/index.js
index 03e1c12..7928e0b 100644
--- a/node_modules/apollo-link-jwt/lib/index.js
+++ b/node_modules/apollo-link-jwt/lib/index.js
@@ -7,6 +7,17 @@ import { addRefreshTokenToBody } from './add-refresh-token-to-body';
 import { isJwtExpired } from './is-jwt-expired';
 import { fetchData } from './fetch-data';
 
+/**
+ * Global cached tokens to prevent an expensive Async lookup on every request
+ */
+let cachedAccessToken = null;
+let cachedRefreshToken = null;
+
+export const forceClearCache = () => {
+  cachedAccessToken = null;
+  cachedRefreshToken = null;
+}
+
 export const ApolloLinkJWT = ({
   apiUrl,
   getTokens,
@@ -15,12 +26,6 @@ export const ApolloLinkJWT = ({
   onRefreshComplete,
   debugMode = false,
 }) => {
-  /**
-   * Global cached tokens to prevent an expensive Async lookup on every request
-   */
-  let cachedAccessToken = null;
-  let cachedRefreshToken = null;
-
   /**
    * Resets the internally cached values
    */