fix #15 (invalid soql building from $exists operator)
fix #15 (invalid soql building from $exists operator)

nutthawan/jsforce,lkatney/jsforce,pcon/jsforce,nutthawan/jsforce,parthiban019/jsforce,jsforce/jsforce,lkatney/jsforce,jesperkristensen/jsforce,joeferraro/jsforce,kimchouard/jsforce,jsforce/jsforce,saleswise/jsforce,saleswise/jsforce,bhaskarmelkani/jsforce,pcon/jsforce,polidog/jsforce,bhaskarmelkani/jsforce,stomita/node-salesforce,jdcrensh/jsforce,parthiban019/jsforce,joeferraro/jsforce,rdietrick/jsforce,pcon/jsforce,jesperkristensen/jsforce,jsforce/jsforce,saleswise/jsforce,jdcrensh/jsforce