acu90005 - fix serialization to json for Bignum

in order to fix 'TypeError: allocator undefined for Bignum'
when we dump hash with Bignum value {:t => 11111111111111}
we don't need to transfor it to structure {"_ruby_class":"Bignum"}.
acu90005 implement code review feedback
Merge pull request #89 from rightscale/purpleKHR_acu90005__fix_serialization_to_json_for_bignum

acu90005 - fix serialization to json for Bignum