UndefVarError when `prepare!` called on Corpus

[tanmaykm]

An UndefVarError is thrown when prepare! is called on a corpus with strip_whitespace option.

julia> prepare!(crps, strip_html_tags | strip_whitespace | strip_non_letters)
ERROR: UndefVarError: d not defined
Stacktrace:
 [1] #prepare!#12(::Set{AbstractString}, ::Set{AbstractString}, ::Function, ::Corpus{Union{FileDocument, NGramDocument, StringDocument, TokenDocument}}, ::UInt32) at /home/tan/.julia/dev/TextAnalysis/src/preprocessing.jl:469
 [2] prepare!(::Corpus{Union{FileDocument, NGramDocument, StringDocument, TokenDocument}}, ::UInt32) at /home/tan/.julia/dev/TextAnalysis/src/preprocessing.jl:458
 [3] top-level scope at none:0

Looks like a typo (copy-paste related?) here:

TextAnalysis.jl/src/preprocessing.jl

Line 417 in 06e85ef

((flags & strip_whitespace) > 0) && remove_whitespace!(d)

introduced in #141

[aviks]

cc: @asbisen