Fix failing test due to quotes handling

Introduced by 7d4c9e53497bd5f0bf1594677d9be8021a1379de. 'in' does
not consider that a Uint8 is in a Vector{Char}, we need to convert
to a Vector{Uint8} first.

Also stop passing 'o.quotemark' to bytestostring() since it is not
used.


Cf. https://github.com/JuliaStats/DataFrames.jl/commit/7d4c9e53497bd5f0bf1594677d9be8021a1379de#commitcomment-4564869