Fix test errors and warnings

[parroty]

Changes

1. Fix for occasional error due to different ordering of multiple headers.

  1) test remove blacklisted headers (ExVCR.RecorderHttpcTest)
     test/recorder_httpc_test.exs:116
     Assertion with == failed
     code:  assert headers == [{'server', 'Cowboy'}, {'content-length', '13'}]
     left:  [{'content-length', '13'}, {'server', 'Cowboy'}]
     right: [{'server', 'Cowboy'}, {'content-length', '13'}]
     stacktrace:
       test/recorder_httpc_test.exs:120: (test)

2. Fix for Kernel.to_char_list/1 deprecation warnings.

Generated exvcr app
warning: Kernel.to_char_list/1 is deprecated, use Kernel.to_charlist/1 instead
  test/adapter_httpc_test.exs:21: ExVCR.Adapter.HttpcTest."test passthrough works when CurrentRecorder has an initial state"/1

warning: Kernel.to_char_list/1 is deprecated, use Kernel.to_charlist/1 instead
  test/adapter_httpc_test.exs:28: ExVCR.Adapter.HttpcTest."test passthrough works after cassette has been used"/1

[coveralls]

Coverage decreased (-0.2%) to 92.308% when pulling 7be7203 on fix-test-errors-and-warnings into 63d47e3 on master.

[coveralls]

Coverage decreased (-0.2%) to 92.308% when pulling 7be7203 on fix-test-errors-and-warnings into 63d47e3 on master.