added merging allele_string for dbSNP variation if having same location
added merging allele_string for dbSNP variation if having same location

dbolser-ebi/ensembl-variation,dbolser/ensembl-variation,willmclaren/ensembl-variation,Ensembl/ensembl-variation,willmclaren/ensembl-variation,dbolser/ensembl-variation,dbolser-ebi/ensembl-variation,dbolser/ensembl-variation,Ensembl/ensembl-variation,dbolser/ensembl-variation,Ensembl/ensembl-variation,willmclaren/ensembl-variation,willmclaren/ensembl-variation,willmclaren/ensembl-variation,dbolser-ebi/ensembl-variation,Ensembl/ensembl-variation,Ensembl/ensembl-variation