Fix CSV.Chunks crashing due to finalized buffer

Fixes #734. The issue here is that on windows, we aggressively finalize
the file source buffer as a way to "close" the file, since windows is
much more picky when trying to modify an mmapped file after parsing.
i.e. windows complains if you try to modify a file and there's still an
mmap reference to it somewhere, so at the end of CSV.File, we call
`finalize(buf)` to ensure it gets unmapped. The problem, however, is
that `CSV.Chunks` calls `CSV.File` multiple times on the same `buf`,
providing different starting/ending byte positions to read in chunks.
So naturally, after the first chunk was read, the buffer was getting
finalized and then the 2nd chunk immediately crashed trying to read its
chunk from a finalized buffer.

src/chunks.jl

@@ -81,7 +81,7 @@ end
 
 function Base.iterate(x::Chunks, i=1)
     i >= length(x.ranges) && return nothing
-    f = File(x.h; startingbyteposition=x.ranges[i], endingbyteposition=(x.ranges[i + 1] - (i != length(x.ranges))), threaded=x.threaded, typemap=x.typemap, tasks=x.tasks, debug=x.debug)
+    f = File(x.h; finalizebuffer=false, startingbyteposition=x.ranges[i], endingbyteposition=(x.ranges[i + 1] - (i != length(x.ranges))), threaded=x.threaded, typemap=x.typemap, tasks=x.tasks, debug=x.debug)
     return f, i + 1
 end
 

src/file.jl

@@ -220,6 +220,7 @@ function File(source;
 end
 
 function File(h::Header;
+    finalizebuffer=true,
     startingbyteposition=nothing,
     endingbyteposition=nothing,
     limit::Union{Integer, Nothing}=nothing,
@@ -344,7 +345,7 @@ function File(h::Header;
     lookup = Dict(k => v for (k, v) in zip(h.names, columns))
     # for windows, it's particularly finicky about throwing errors when you try to modify an mmapped file
     # so we just want to make sure we finalize the input buffer so users don't run into surprises
-    if Sys.iswindows() && ncols > 0 && !lazystrings(flags[1])
+    if finalizebuffer && Sys.iswindows() && ncols > 0 && !lazystrings(flags[1])
         finalize(buf)
     end
     return File{something(threaded, false)}(h.name, h.names, types, finalrows, ncols, columns, lookup)

src/header.jl

@@ -38,7 +38,7 @@ Checks whether a character or string is valid for use as a delimiter.  If
 Throws an error if `delim` is invalid.
 """
 function checkvaliddelim(delim)
-    delim != nothing && !isvaliddelim(delim) &&
+    delim !== nothing && !isvaliddelim(delim) &&
         throw(ArgumentError("invalid delim argument = '$(escape_string(string(delim)))', "*
                             "the following delimiters are invalid: '\\r', '\\n', '\\0'"))
 end