Fix CurrentRecorder initial state

[surik]

The initial state should be nil to properly bypass mock when no cassette is used.

See #159 (comment)

[coveralls]

Coverage increased (+0.2%) to 92.491% when pulling cf9585a on surik:fix-ccurrent-recorder-initial-state into d757ad4 on parroty:master.

[coveralls]

Coverage remained the same at 92.308% when pulling cf9585a on surik:fix-ccurrent-recorder-initial-state into d757ad4 on parroty:master.

[parroty]

Thanks!

[parroty]

@surik
Just one thing that following warning seems emitted during the build (though the code itself does not appear multiple times). Does it make sense to change _arg to _?

• https://github.com/parroty/exvcr/pull/163/files#diff-74e7d0b80f0c5ac53252d45c8db73693a00383ebedede7b3b5ecbb5ef7339133R58

$ mix test
warning: the underscored variable "_arg" appears more than once in a match. This means the pattern will only match if all "_arg" bind to the same value. If this is the intended behaviour, please remove the leading underscore from the variable name, otherwise give the variables different names
  lib/exvcr/actor.ex:58: ExVCR.Actor.CurrentRecorder.start_link/1

warning: the underscored variable "_arg" is used after being set. A leading underscore indicates that the value of the variable should be ignored. If this is intended please rename the variable to remove the underscore
  lib/exvcr/actor.ex:58: ExVCR.Actor.CurrentRecorder.start_link/1

$ elixir -v
Erlang/OTP 23 [erts-11.0.3] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe] [dtrace]

Elixir 1.11.2 (compiled with Erlang/OTP 23)

[surik]

@parroty ah yes, this would make sense.

[parroty]

Thanks.