Fixes deprecation warning:
```
DEPRECATION WARNING: The success? predicate is deprecated and
will be removed in Rails 6.0.
Please use successful? as provided by Rack::Response::Helpers.
```
This enables FindCommit caching to the following actions:
* BlobController#show
* RefsController#logs_tree
It also improves caching in CommitsController since some
duplicate requests were occuring inside the before_action
definitions.
Updates specs to use new rails5 format.
The old format:
`get :show, { some: params }, { some: headers }`
The new format:
`get :show, params: { some: params }, headers: { some: headers }`