Wire up acquire_token_silent(..., force_refresh=...)

Interestingly, during last 10 months since this library was first released,
there has been basically no customer found and asked for this feature
(except only once, but that one was a red herring so he did not really want it
#56 (comment)
).

Author: rayluo

msal/application.py

@@ -414,7 +414,7 @@ def acquire_token_silent(
                 validate_authority=False,
                 verify=self.verify, proxies=self.proxies, timeout=self.timeout)
             result = self._acquire_token_silent_from_cache_and_possibly_refresh_it(
-                scopes, account, the_authority, **kwargs)
+                scopes, account, the_authority, force_refresh=force_refresh, **kwargs)
             if result:
                 return result