Oj expects indent as a Fixnum.

When using multi_json with Oj, and also the json lib happens to be laoded, there's a case the json lib sets indent as a String (see https://github.com/ruby/ruby/blob/v2_1_0/ext/json/lib/json/common.rb), making Oj crash with ArgumentError: :indent must be a Fixnum.
Merge pull request #145 from oriolgual/patch-1

[Fix] Oj expects indent as a Fixnum