Update documentation for SequenceServer#to_fasta.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>
Ensure Sequenceserver#to_fasta doesn't change the sequence in place.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>
Make SequenceServer#to_fasta's spec more stringent.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>