Always convert data to data.frame for arrange_venn

[maximilian-heeg]

• In the arrange_venn function, the data is always converted to a data.frame. This is necessary to reorder the metadata columns if for example a tibble is used.
• Adds a test to verify that arrange_venn works with tibbles.

Closed #116

[codecov-commenter]

Codecov Report

Merging #159 (98310fc) into master (7134b90) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #159   +/-   ##
=======================================
  Coverage   96.12%   96.12%           
=======================================
  Files           4        4           
  Lines        1548     1549    +1     
=======================================
+ Hits         1488     1489    +1     
  Misses         60       60           

Impacted Files	Coverage Δ
R/venn.R	92.18% <100.00%> (+0.02%)	:arrow_up:

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