Fix typo in error subscription in Observable#do

lib/rx/linq/observable/do.rb

@@ -22,7 +22,7 @@ def do(observer_or_on_next = nil, on_error_func = nil, on_completed_func = nil)
           },
           lambda {|err|
             begin
-              on_error_func && on_error_func.call(x)
+              on_error_func && on_error_func.call(err)
             rescue => e
               observer.on_error e
             end

test/rx/linq/observable/test_do.rb

@@ -0,0 +1,16 @@
+require "#{File.dirname(__FILE__)}/../../../test_helper"
+
+class TestObservableDo < Minitest::Test
+  def test_do_error_propagation
+    expected = RuntimeError.new
+    actual = nil
+    assert_raises(RuntimeError) do
+      Rx::Observable.raise_error(expected).do(
+        ->(_) {},
+        ->(_err) { actual = _err },
+        -> {}
+      ).subscribe
+    end
+    assert_equal actual, expected
+  end
+end