improper stemming of NGram documents

Stemming a NGramDocument stems only the last word of each ngram. Notice below how `repository` is stemmed to `repositori` in one place but left intact in another.
```
julia> td = NGramDocument("this repository of julia language", 3)
NGramDocument{AbstractString}(Dict{AbstractString,Int64}("language"=>1,"repository"=>1,"this"=>1,"this repository of"=>1,"of julia language"=>1,"julia language"=>1,"of"=>1,"julia"=>1,"this repository"=>1,"repository of"=>1…), 3, TextAnalysis.DocumentMetadata(Languages.English(), "Untitled Document", "Unknown Author", "Unknown Time"))

julia> stem!(td); td
NGramDocument{AbstractString}(Dict{AbstractString,Int64}("languag"=>1,"this"=>1,"this repository of"=>1,"of julia languag"=>1,"this repositori"=>1,"of"=>1,"julia"=>1,"repositori"=>1,"repository of"=>1,"of julia"=>1…), 3, TextAnalysis.DocumentMetadata(Languages.English(), "Untitled Document", "Unknown Author", "Unknown Time"))
```

While stemming a StringDocument stems each word:
```
julia> sd = StringDocument("this repository of julia language")
StringDocument{String}("this repository of julia language", TextAnalysis.DocumentMetadata(Languages.English(), "Untitled Document", "Unknown Author", "Unknown Time"))

julia> stem!(sd); sd
StringDocument{String}("this repositori of julia languag", TextAnalysis.DocumentMetadata(Languages.English(), "Untitled Document", "Unknown Author", "Unknown Time"))
```