Relax type constraints on filenames

[oxinabox]

To allow this to work with FilePathBase file types.
Which no longer subtype AbstractString (rofinn/FilePathsBase.jl#22)

This change basically says tht if you don't give it a IO object,
then whatever you do give it must have an open method.

[codecov]

Codecov Report

Merging #394 into master will decrease coverage by 13.19%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #394      +/-   ##
==========================================
- Coverage   76.25%   63.05%   -13.2%     
==========================================
  Files           8        8              
  Lines        1040     1053      +13     
==========================================
- Hits          793      664     -129     
- Misses        247      389     +142

Impacted Files	Coverage Δ
src/CSV.jl	69.15% <66.66%> (-7.7%)	⬇️
src/tables.jl	65.09% <0%> (-27.22%)	⬇️
src/transforms.jl	48.57% <0%> (-16.95%)	⬇️
src/write.jl	80.85% <0%> (-15.96%)	⬇️
src/filedetection.jl	66.66% <0%> (-13.23%)	⬇️
src/deprecated.jl	48.16% <0%> (-11.15%)	⬇️
src/typedetection.jl	83.33% <0%> (-9.17%)	⬇️
src/validate.jl	90% <0%> (-5%)	⬇️

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[quinnj]

Closing in favor of #405