Add pretty_generate coverage, skip Oj

Oj's pretty generate capabilities currently can't match the output
of the `json` gem. The `json` gem formats the JSON in a pretty
inconsistent and ugly way, but we have a number of specs that expect
it, and so I've added test coverage to ensure it matches.

Because of this, Oj usage is currently disabled for .pretty_generate.

lib/gitlab/json.rb

@@ -57,25 +57,14 @@ module Gitlab
       # The Oj variant in this looks seriously weird but these are the settings
       # needed to emulate the style generated by the JSON gem.
       #
+      # NOTE: This currently ignores Oj, because Oj doesn't generate identical
+      #       formatting, issue: https://github.com/ohler55/oj/issues/608
+      #
       # @param object [Hash, Array, Object] must be hash, array, or an object that responds to .to_h or .to_json
       # @param opts [Hash] an options hash with fewer supported settings than .dump
       # @return [String]
       def pretty_generate(object, opts = {})
-        if enable_oj?
-          adapter_generate(
-            object,
-            {
-              indent: "  ",
-              space: " ",
-              object_nl: "\n",
-              array_nl: "\n"
-            }.merge(opts)
-          ).chomp
-        else
-          opts = standardize_opts(opts)
-
-          ::JSON.pretty_generate(object, opts)
-        end
+        ::JSON.pretty_generate(object, opts)
       end
 
       private

spec/lib/gitlab/json_spec.rb

@@ -217,7 +217,15 @@ RSpec.describe Gitlab::Json do
 
     describe ".pretty_generate" do
       let(:obj) do
-        { test: true, "foo.bar" => "baz", is_json: 1, some: [1, 2, 3] }
+        {
+          test: true,
+          "foo.bar" => "baz",
+          is_json: 1,
+          some: [1, 2, 3],
+          more: { test: true },
+          multi_line_empty_array: [],
+          multi_line_empty_obj: {}
+        }
       end
 
       it "generates pretty JSON" do
@@ -230,7 +238,15 @@ RSpec.describe Gitlab::Json do
               1,
               2,
               3
-            ]
+            ],
+            "more": {
+              "test": true
+            },
+            "multi_line_empty_array": [
+
+            ],
+            "multi_line_empty_obj": {
+            }
           }
         STR
 
@@ -253,7 +269,15 @@ RSpec.describe Gitlab::Json do
               1,
               2,
               3
-            ]
+            ],
+            "more" : {
+              "test" : true
+            },
+            "multi_line_empty_array" : [
+
+            ],
+            "multi_line_empty_obj" : {
+            }
           }
         STR