Restored start_iteration [skip ci]

Author: ankane

lib/lightgbm/booster.rb

@@ -160,7 +160,7 @@ def predict(input, start_iteration: nil, num_iteration: nil, **params)
       singular = !input.first.is_a?(Array)
       input = [input] if singular
 
-      start_iteration = 0
+      start_iteration ||= 0
       num_iteration ||= best_iteration
       num_class = self.num_class