Fix export crash for JSON fields

NoMethodError:
undefined method `content_tag' for nil:NilClass
<class:Json>'

Author: blaze182

lib/rails_admin/config/fields/types/json.rb

@@ -17,6 +17,10 @@ class Json < RailsAdmin::Config::Fields::Types::Text
             bindings[:view].content_tag(:pre) { formatted_value }.html_safe
           end
 
+          register_instance_option :export_value do
+            formatted_value
+          end
+
           def parse_value(value)
             value.present? ? JSON.parse(value) : nil
           rescue JSON::ParserError

spec/integration/basic/export/rails_admin_basic_export_spec.rb

@@ -46,7 +46,8 @@
       click_button 'Export to csv'
       csv = CSV.parse page.driver.response.body.force_encoding('utf-8') # comes through as us-ascii on some platforms
       expect(csv[0]).to match_array ['Id', 'Created at', 'Updated at', 'Deleted at', 'Name', 'Position',
-                                     'Number', 'Retired', 'Injured', 'Born on', 'Notes', 'Suspended', 'Formation', 'Id [Team]', 'Created at [Team]',
+                                     'Number', 'Retired', 'Injured', 'Born on', 'Notes', 'Suspended', 'Formation', 'Schedule',
+                                     'Id [Team]', 'Created at [Team]',
                                      'Updated at [Team]', 'Name [Team]', 'Logo url [Team]', 'Team Manager [Team]', 'Ballpark [Team]',
                                      'Mascot [Team]', 'Founded [Team]', 'Wins [Team]', 'Losses [Team]', 'Win percentage [Team]',
                                      'Revenue [Team]', 'Color [Team]', 'Custom field [Team]', 'Main Sponsor [Team]', 'Id [Draft]', 'Created at [Draft]',

spec/rails_admin/config/fields/types/json_spec.rb

@@ -58,6 +58,33 @@
     end
   end
 
+  describe '#export_value' do
+    before do
+      RailsAdmin.config do |config|
+        config.model FieldTest do
+          field :json_field, :json
+        end
+      end
+    end
+
+    it 'returns correct value for empty json' do
+      allow(object).to receive(:json_field) { {} }
+      actual = field.with(bindings).export_value
+      expect(actual).to match(/{\n+}/)
+    end
+
+    it 'returns correct value' do
+      allow(object).to receive(:json_field) { {sample_key: "sample_value"} }
+      actual = field.with(bindings).export_value
+      expected = [
+        "{",
+        "  \"sample_key\": \"sample_value\"",
+        "}",
+      ].join("\n")
+      expect(actual).to eq(expected)
+    end
+  end
+
   describe '#parse_input' do
     before :each do
       RailsAdmin.config do |config|
@@ -72,8 +99,8 @@
       expect(field.parse_input(json_field: data.to_json)).to eq data
     end
 
-    it 'raise JSON::ParserError with invalid json string' do
-      expect { field.parse_input(json_field: '{{') }.to raise_error(JSON::ParserError)
+    it 'return nil with invalid json string' do
+      expect(field.parse_input(json_field: '{{')).to be_nil
     end
   end