Update tfidf.js
Update tfidf.js

Fixing problem when deriving tfidf for the term 'constructor' https://github.com/NaturalNode/natural/issues/119
ahmedshuhel/natural,moos/natural,fergiemcdowall/natural,mcanthony/natural,kjhenner/natural,fergiemcdowall/natural,levy5674/natural,fergiemcdowall/natural,PTaylour/natural,Logicify/natural,karimsa/natural,itamayo/natural,PMSI-AlignAlytics/natural,NaturalNode/natural,polymaths/natural,dpraimeyuu/natural,walkingdoctors/natural,mamaral/natural,Hugo-ter-Doest/natural