recompute id2word after updating Dictionary with new docs
recompute id2word after updating Dictionary with new docs

`id2word` has "lazy" semantics (computed only when asked to), it acts like a cache.
Previously the cache never got updated; now it's updated whenever the Dictionary changes and `id2word` is requested again.

tzoiker/gensim,mattilyra/gensim,pombredanne/gensim,gojomo/gensim,olavurmortensen/gensim,zachmayer/gensim,gojomo/gensim,quole/gensim,macks22/gensim,mattilyra/gensim,olavurmortensen/gensim,pombredanne/gensim,mattilyra/gensim,olavurmortensen/gensim,ELind77/gensim,bhargavvader/gensim,gojomo/gensim,davechallis/gensim,manasRK/gensim,quole/gensim,ziky90/gensim,bhargavvader/gensim,RaRe-Technologies/gensim,quole/gensim,markroxor/gensim,ziky90/gensim,Kreiswolke/gensim,loisaidasam/gensim,robotcator/gensim,dsquareindia/gensim,summanlp/gensim,isohyt/gensim,bhargavvader/gensim,Kreiswolke/gensim,tzoiker/gensim,ELind77/gensim,akutuzov/gensim,summanlp/gensim,dsquareindia/gensim,markroxor/gensim,zachmayer/gensim,summanlp/gensim,macks22/gensim,RaRe-Technologies/gensim,zachmayer/gensim,robotcator/gensim,RaRe-Technologies/gensim,RaRe-Technologies/gensim,piskvorky/gensim,akutuzov/gensim,akutuzov/gensim,davechallis/gensim,piskvorky/gensim,davechallis/gensim,laic/gensim,markroxor/gensim,pombredanne/gensim,dsquareindia/gensim,manasRK/gensim,laic/gensim,jaksmid/gensim,laic/gensim,loisaidasam/gensim,macks22/gensim,gojomo/gensim,Kreiswolke/gensim,ziky90/gensim,ELind77/gensim,isohyt/gensim,jaksmid/gensim,manasRK/gensim,jaksmid/gensim,robotcator/gensim,tzoiker/gensim,isohyt/gensim,loisaidasam/gensim,piskvorky/gensim