ARROW-6323: [R] Expand file paths when passing to readers

[nealrichardson]

Note that there is no test in the test suite that asserts that ~ is expanded correctly because we can only write to tempfiles in the tests, but It Works on My Machine ™️ .

cc @romainfrancois

[nealrichardson]

Debugging the windows build failure, stand by

[codecov-io]

Codecov Report

Merging #5169 into master will decrease coverage by 12.41%.
The diff coverage is 83.33%.

@@             Coverage Diff             @@
##           master    #5169       +/-   ##
===========================================
- Coverage   87.63%   75.22%   -12.42%     
===========================================
  Files        1022       57      -965     
  Lines      146258     3604   -142654     
  Branches     1437        0     -1437     
===========================================
- Hits       128174     2711   -125463     
+ Misses      17722      893    -16829     
+ Partials      362        0      -362

Impacted Files	Coverage Δ
r/R/feather.R	63.41% <ø> (-1.71%)	⬇️
r/R/csv.R	97.22% <ø> (-0.15%)	⬇️
r/R/RecordBatchWriter.R	72.22% <ø> (+2.22%)	⬆️
r/R/json.R	90% <ø> (-1.31%)	⬇️
r/R/read_table.R	100% <ø> (ø)	⬆️
r/R/write_arrow.R	96.42% <ø> (-0.13%)	⬇️
r/R/compression.R	90.9% <ø> (-1.4%)	⬇️
r/R/RecordBatchReader.R	70.58% <ø> (-1.64%)	⬇️
r/R/parquet.R	73.07% <100%> (ø)	⬆️
r/R/io.R	74.28% <75%> (ø)	⬆️
... and 965 more

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[fsaintjacques]

Is it expected from R libraries to expand path? If so, I have no objection to merge this.

[nealrichardson]

Yes:

> write.csv(iris, file="~/Downloads/iris.csv")
> read.csv("~/Downloads/iris.csv")
      X Sepal.Length Sepal.Width Petal.Length Petal.Width    Species
1     1          5.1         3.5          1.4         0.2     setosa
2     2          4.9         3.0          1.4         0.2     setosa
3     3          4.7         3.2          1.3         0.2     setosa
4     4          4.6         3.1          1.5         0.2     setosa
5     5          5.0         3.6          1.4         0.2     setosa
6     6          5.4         3.9          1.7         0.4     setosa
...