fix sample-(neg-)binomial mangling arguments

[lionandoil]

was:

=> (sample-binomial 10) ; expect a seq of 10 0's and 1's
1
=> (sample-binomial 1 :size 10) ; expect a single number between 0 and 10
(4 9 5 7 4 8 4 6 6 6)

is:

=> (sample-binomial 10)
(1 1 1 0 0 1 0 0 1 1)
=> (sample-binomial 1 :size 10)
6

see also #109