Fix issue of passing nil to formatters (from CSV PR)

[PragTob]

Interestingly the bug occurred thanks to our legacy format support.
Can't wait to get rid off it ;)

I changed the code for the formatter test to really make sure that
the same options passed to format/2 are also passed to write/2.
It didn't find the bug as the bug was in Configuration, but I
think it's still an improvement worth keeping.

Once this code is merged, we can delet the one unnecessary line
from the CSV formatter. Already tried it and it worked 🎉

[PragTob]

Falkyness:

  1) test .run_scenarios very fast function times are almost 0 with function call overhead elimination (Benchee.Benchmark.RunnerTest)
     test/benchee/benchmark/runner_test.exs:184
     Assertion with <= failed
     code:  assert median <= 60
     left:  183.0
     right: 60
     stacktrace:
       test/benchee/benchmark/runner_test.exs:196: (test)

Sad, restarted