Eagerload active_support/json/encoding in active_support/core_ext/object/to_json 

This is related to #12106. (The root cause for that ticket is that json/add defines Regexp#to_json among others, but here I'll reproduce the problem without json/add.)

Before:

```ruby
>> require 'active_support/core_ext'
=> true
>> //.as_json
NoMethodError: undefined method `as_json' for //:Regexp
  from (irb):3
  from /Users/godfrey/.rvm/rubies/ruby-2.0.0-p195/bin/irb:16:in `<main>'
>> //.to_json
=> "\"(?-mix:)\""
>> //.as_json
=> "(?-mix:)"
```

After:

```ruby
>> require 'active_support/core_ext'
=> true
>> //.as_json
=> "(?-mix:)"
```

When someone require 'active_support/core_ext', the expectation is that it would add certain methods to the core classes NOW. The previous behaviour causes additional methods to be loaded the first time you call `to_json`, which could cause nasty surprises.