fix the guard for max amount of serializers

[juanperi]

While reviewing #45, I just spotted that the guard is wrong, as allows up until 8 serializers.
At the same time, the test was wrong, as adding up until 10 serializers more doesn't ensure that we can have up to 7

[coveralls]

Coverage remained the same at 100.0% when pulling 68709fd on epilgrim:fix/amount-of-serializers-guard into 553d452 on sorentwo:master.

[sorentwo]

Thanks for catching these (and for helping with the rassoc fix in the other PR). Merged locally, closed with f336167.