auth/imap: close IMAP connection earlier

No need to keep it around after Authenticate succeeds while handling
the HTTP request (via next.ServeHTTP).

auth/imap.go

@@ -57,6 +57,7 @@ func (prov *IMAPProvider) doAuth(next http.Handler,
 		http.Error(w, "Invalid username or password", http.StatusUnauthorized)
 		return
 	}
+	conn.Close()
 
 	authCtx := AuthContext{
 		AuthMethod: "imap",