Test current behavior regarding depth for Coder

Coder currently ignores its depth and always resets it to 0 when
generating a new JSON document.

Author: etiennebarrie

test/json/json_coder_test.rb

@@ -132,6 +132,11 @@ def test_json_coder_string_invalid_encoding
     assert_equal 2, calls
   end
 
+  def test_depth
+    coder = JSON::Coder.new(object_nl: "\n", array_nl: "\n", space: " ", indent: "  ", depth: 1)
+    assert_equal %({\n  "foo": 42\n}), coder.dump(foo: 42)
+  end
+
   def test_nesting_recovery
     coder = JSON::Coder.new
     ary = []