fix CachedDataset breaking precompilation

[CarloLucibello]

Fixing broken precompilation

julia> using MLDatasets
[ Info: Precompiling MLDatasets [eb30cadb-4394-5ae3-aed4-317e484a6458]
WARNING: Method definition (::Type{MLDatasets.CachedDataset{T, S} where S where T})(Any) in module MLDatasets at /home/carlo/.julia/dev/MLDatasets/src/containers/cacheddataset.jl:29 overwritten at /home/carlo/.julia/dev/MLDatasets/src/containers/cacheddataset.jl:31.
  ** incremental compilation may be fatally broken for this module **

[codecov-commenter]

Codecov Report

Merging #102 (07975ff) into master (42cc48f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #102   +/-   ##
=======================================
  Coverage   36.02%   36.02%           
=======================================
  Files          38       38           
  Lines         880      880           
=======================================
  Hits          317      317           
  Misses        563      563           

Impacted Files	Coverage Δ
src/containers/cacheddataset.jl	100.00% <100.00%> (ø)

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