Resolve circular reference issue while generating JSON with Oj

[dgynn]

This PR addresses the problem identified in #70 where oj_mimic_json prevents the JSON response from being created.

Oj prefers as_json to to_json and previously the TimerStruct classes only implemented to_json. This commit adds as_json support that matches the to_json implementation so either way will work.

Specifically, the circular reference is between Page and Request (TimerStruct) objects where Page has reference to a @attributes[:root] Request and Request has a @page attribute.

I have not added test cases for this new code because as_json will only work if ActiveSupport extensions have loaded so that Hash has as_json support. The fix can be verified by adding the oj_mimic_json gem to an existing app.

[codecov-io]

Current coverage is 84.04%

Merging #250 into master will decrease coverage by 0.10%

@@             master       #250   diff @@
==========================================
  Files            21         21          
  Lines          1161       1166     +5   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits            977        980     +3   
- Misses          184        186     +2   
  Partials          0          0          

Powered by Codecov. Last updated by 770665a...83f65b0

[SamSaffron]

cool!

[kbrock]

Thanks so much. Many people wanted to get OJ working