Added spec demonstrating issue #665 (References not getting garbage collected)

This PR contains failing specs that demonstrate scenarios when references `Grape::Endpoint` are not collected when running the tests. In writing these specs I noticed that adding a second call to a spec that produces a 404 seems to clear out the bad reference. A second call in the same spec that produces a 200 does not.

Also note that in each scenario references to `Regexp` were either getting collected or not.

```
Failures:

  1) Grape::API when performing a single call against the endpoint is adding an uncollectable reference to Grape::Endpoint
     Failure/Error: expect(post_test_counts).to eql pre_test_counts

       expected: {"Grape::Endpoint"=>48, "Grape::Util::HashStack"=>64, "Regexp"=>2164}
            got: {"Grape::Endpoint"=>49, "Grape::Util::HashStack"=>64, "Regexp"=>2166}

       (compared using eql?)

       Diff:

       @@ -1,4 +1,4 @@
       -"Grape::Endpoint" => 48,
       +"Grape::Endpoint" => 49,
        "Grape::Util::HashStack" => 64,
       -"Regexp" => 2164
       +"Regexp" => 2166

     # ./spec/grape/garabge_collection_spec.rb:53:in `block (3 levels) in <top (required)>'

  2) Grape::API when performing multiple calls against the endpoint is dropping that extra reference to Grape::Endpoint
     Failure/Error: expect(post_test_counts).to eql pre_test_counts

       expected: {"Grape::Endpoint"=>48, "Grape::Util::HashStack"=>64, "Regexp"=>2166}
            got: {"Grape::Endpoint"=>48, "Grape::Util::HashStack"=>64, "Regexp"=>2173}

       (compared using eql?)

       Diff:
       @@ -1,4 +1,4 @@
        "Grape::Endpoint" => 48,
        "Grape::Util::HashStack" => 64,
       -"Regexp" => 2166
       +"Regexp" => 2173

     # ./spec/grape/garabge_collection_spec.rb:70:in `block (3 levels) in <top (required)>'

  3) Grape::API when performing multiple calls against the endpoint is not dropping that extra reference to Grape::Endpoint
     Failure/Error: expect(post_test_counts).to eql pre_test_counts

       expected: {"Grape::Endpoint"=>48, "Grape::Util::HashStack"=>64, "Regexp"=>2173}
            got: {"Grape::Endpoint"=>49, "Grape::Util::HashStack"=>64, "Regexp"=>2173}

       (compared using eql?)

       Diff:
       @@ -1,4 +1,4 @@
       -"Grape::Endpoint" => 48,
       +"Grape::Endpoint" => 49,
        "Grape::Util::HashStack" => 64,
        "Regexp" => 2173
     # ./spec/grape/garabge_collection_spec.rb:85:in `block (3 levels) in <top (required)>'

Finished in 4.13 seconds
754 examples, 3 failures
```