topk's Query() returns the full sample slice if the slice length is less than k

[bmizerany]

Do you mind adding a test to reproduce the problem and I'll merge this in?

[daneharrigan]

@bmizerany there you go. thanks for the reminder

[bmizerany]

Do you mind re-writing it as a table-driven test? I'd like to get away from the old way.

[daneharrigan]

@bmizerany updated to a table-driven test. let me know if i've missed anything else

[bmizerany]

Thanks! Rebased and merged.