secret_old_rest in rest-graph

ok, I realized that the broken JSON Facebook has returned,
could be viewed as double encoded, so I've made an option
called double_decode addition to suppress_decode for ALL
API calls including get/post/old_rest, etc.

That is, if we know the broken JSON is double encoded,
we could pass double_decode => true to properly decode it.
Or, just pass suppress_decode => true to not parse it at all.

Also, I still keep the broken_old_rest there. It's simply calling
secret_old_rest with double_decode => true on by default.

secret_old_rest changes, and the documentation changes.

Sorry, I still didn't feel right to do auto-detect, nor to list
all the experiments result neither. We can't 100% be sure it's
properly detected, since the broken JSON could be viewed as a
really JSON as well. For example, yajl-ruby (a better JSON parse
for Ruby) could parse it... But not for the most seen json gem.

Better not trying to be too smart, isn't it? :p