fix rack mini profiler backtraces
fix rack mini profiler backtraces

Test plan:
  * enable profiling (?pp=enable)
  * click on an sql query to get more details
  * you should see backtraces with each query

Change-Id: I8019ee0f5be2b1c3870e5d7b0f37eae0d5889f07
Reviewed-on: https://gerrit.instructure.com/67706
Reviewed-by: Keith T. Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>
Tested-by: Jenkins
QA-Review: Jason Carter <976bfc4bed0555ff96b3266aea18e7b17a9d471c@instructure.com>
Product-Review: Cameron Matheson <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>

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