Raise error for duplicate headers in hash mode (fixes #44)

[martijn]

As discussed in #44 the current behaviour for hash mode has the potential leads to confusion.

This change makes Xsv raise an error when trying to enter hash mode on a sheet with duplicate non-nil header values. I left nil out of scope because it's common for Excel sheets to have empty columns for spacing or trailing on the end of the sheet.

I'm not sure if we really want this behaviour, and I'm planning on bumping the minor version to signal the potential breakage this can cause. @shkm @voxik looking forward to your comments on this one.

[codecov-commenter]

Codecov Report

Merging #45 (c8859f7) into main (420dadb) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #45   +/-   ##
=======================================
  Coverage   99.71%   99.71%           
=======================================
  Files          21       21           
  Lines         692      707   +15     
=======================================
+ Hits          690      705   +15     
  Misses          2        2           

Impacted Files	Coverage Δ
lib/xsv.rb	100.00% <100.00%> (ø)
lib/xsv/sheet.rb	100.00% <100.00%> (ø)
test/sheet_test.rb	100.00% <100.00%> (ø)

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[voxik]

Although it does not address the #44, I like this 👍 I think it is better to fail then possibly deliver corrupted data.

[shkm]

+1 but I think it's worth making that change so we don't have to look up zip again ;-).

[martijn]

+1 but I think it's worth making that change so we don't have to look up zip again ;-).

Ohh yeah that is much better, thanks!