SpacedKmer bug for 2bit sequences

[cjprybol]

# generate random 4bit sequence
randseq_4bit = BioSequences.randdnaseq(300)
@assert typeof(randseq_4bit) == BioSequences.LongDNA{4}

# convert sequence to 2bit
randseq_2bit = BioSequences.LongDNA{2}(randseq_4bit)
@assert typeof(randseq_2bit) == BioSequences.LongDNA{2}

@assert randseq_2bit == randseq_4bit

# compare taking every 3rd every-mer against spaced kmers of stride 3
# returns true
collect(Kmers.EveryKmer{Kmers.DNACodon}(randseq_4bit))[1:3:end] == collect(Kmers.SpacedKmers{Kmers.DNACodon}(randseq_4bit, 3))

# returns false
collect(Kmers.EveryKmer{Kmers.DNACodon}(randseq_2bit))[1:3:end] == collect(Kmers.SpacedKmers{Kmers.DNACodon}(randseq_2bit, 3))