fix bug in self-avoiding sampling

Tiny bug fix

```
>a = rep("blabla", 1000)
>sample(a,7; replace=false)

no method convert(Type{ASCIIString}, Int64)
while loading In[111], in expression starting on line 2
 in setindex! at dict.jl:517
 in self_avoid_sample! at /home/nikos/.julia/StatsBase/src/sampling.jl:60
 in sample! at /home/nikos/.julia/StatsBase/src/sampling.jl:141
 in sample at /home/nikos/.julia/StatsBase/src/sampling.jl:152
```