Fix wrong max retry timeout for exponential backoff again

The patch in #3640 was still wrong, the expected one is

  c + c * b^1 + (...) + c*b^k

but the patch was

  c + c * b^1 + (...) + c*b^(k-1)

Signed-off-by: Takuro Ashie <[email protected]>

Author: ashie

lib/fluent/plugin_helper/retry_state.rb

@@ -158,7 +158,7 @@ def naive_next_time(retry_next_times)
 
         def calc_max_retry_timeout(max_steps)
           result = 0
-          max_steps.times { |i|
+          (0..max_steps).each { |i|
             result += calc_interval(i + 1)
           }
           result

test/plugin_helper/test_retry_state.rb

@@ -413,7 +413,7 @@ class Dummy < Fluent::Plugin::TestBase
     override_current_time(s, dummy_current_time)
 
     timeout = 0
-    5.times { |i| timeout += 1.0 * (2 ** i) }
+    (0..5).each { |i| timeout += 1.0 * (2 ** i) }
 
     assert_equal dummy_current_time, s.current_time
     assert_equal (dummy_current_time + 100), s.timeout_at