Return full file when doing partial request

[nanjiangshu]

This PR closes https://github.com/GenomicDataInfrastructure/starter-kit-htsget/issues/6

Changes made The htsget server returns the full file when a user request parts of it.

Note that this is just a temporary solution since the partial request functionality is still not ready for the htsget-refserver.

with this change, all the following commands

samtools view  http://localhost:3000/reads/s3/NA12878_20k_b37.bam
samtools view "http://localhost:3000/reads/s3/NA12878_20k_b37.bam?referenceName=1&start=10000&end=12000"
htsget --format bam --output out.bam --reference-name 1 --start 10000 --end 12000 http://localhost:3000/reads/s3/NA12878_20k_b37.bam 

return the complete content of the requested BAM file.

further commens I tried to fix the partial retrieving functionality but it seems not that easy.