Fix: ruby#460

The various `to_json` methods must rescue exceptions
to free the buffer.

```
require 'json'

data = 10_000.times.to_a << BasicObject.new
20.times do
  100.times do
    begin
      data.to_json
    rescue NoMethodError
    end
  end
  puts `ps -o rss= -p #{$$}`
end
```

```
 20128
 24992
 29920
 34672
 39600
 44336
 49136
 53936
 58816
 63616
 68416
 73232
 78032
 82896
 87696
 92528
 97408
102208
107008
111808
```