uft8 source file support

[schloerke]

Fixes: #312

This stems from wanting to read utf8 files on a non utf8 locale. On windows, the locale is set to the system default which typically does not support utf8 out of the box.

utf8.R functions are adopted from the RStudio Shiny repo in an effort to use code that has already be validated to work.

The end result is that source(file, encoding="UTF-8") does not work as expected and instead we should use eval(parse(file, encoding = "UTF-8"))

Other additions

• Use testthat::test_path when loading test files
• Add Barret as ctb
• Removes appveyor.yml build
• Only source all expressions in a plumber file once - cde0d3d

[codecov-io]

Codecov Report

Merging #328 into master will decrease coverage by 1.18%.
The diff coverage is 56.41%.

@@            Coverage Diff             @@
##           master     #328      +/-   ##
==========================================
- Coverage   90.04%   88.86%   -1.19%     
==========================================
  Files          26       27       +1     
  Lines        1145     1176      +31     
==========================================
+ Hits         1031     1045      +14     
- Misses        114      131      +17

Impacted Files	Coverage Δ
R/parse-block.R	96.44% <100%> (+0.01%)	⬆️
R/plumber.R	84.65% <100%> (-0.09%)	⬇️
R/utf8.R	50% <50%> (ø)
R/paths.R	75% <0%> (-1.93%)	⬇️

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[wch]

We now understand why source() was being called. It shouldn't be necessary if activateBlock() is modified to do eval(expr, envir) when there's no plumber metadata associated with the expression.