opal/core: add polyfill for JSON.generate
opal/core: add polyfill for JSON.generate

The builtin JSON functionality in Opal has some weirdness in that it
calls Hash#to_json with an argument, but that function doesn't accept
arguments.  Add a polyfill that simply doesn't provide any arguments to
the appropriate #to_json method.

Signed-off-by: brian m. carlson <738bdd359be778fee9f0fc4e2934ad72f436ceda@crustytoothpaste.net>

bk2204/daniel-ruby,bk2204/daniel-ruby