add rescue_json to examples

Add example, provided by @nickhoffman + short explanation about it.
[Related issue](https://github.com/jnunemaker/httparty/issues/170) (which i think must be closed)
Pretty sure the default rescue is StandardError.

"If you write a rescue clause with no parameter list, the parameter defaults to StandardError."
http://rubylearning.com/satishtalim/ruby_exceptions.html