add excel serializer/parser

[r2evans]

Fixes #973
Fixes #959

PR task list:

• Update NEWS
• Add tests
• Update documentation with devtools::document()

[CLAassistant]

All committers have signed the CLA.

[schloerke]

LGTM! Just missing tests for the serializer and parser

Feel free to duplicate how parquet approached them:

• plumber/tests/testthat/test-parse-body.R

  Lines 112 to 135 in f4230ed

  	test_that("Test parquet parser", {
  	skip_if_not_installed("arrow")
  	tmp <- tempfile()
  	on.exit({
  	file.remove(tmp)
  	}, add = TRUE)
  	r_object <- iris
  	res <- try(arrow::write_parquet(r_object, tmp))
  	skip_if(
  	inherits(res, "try-error"),
  	"arrow::write_parquet() isn't working."
  	)
  	val <- readBin(tmp, "raw", 10000)
  	parsed <- parse_body(val, "application/vnd.apache.parquet", make_parser("parquet"))
  	# convert from parquet tibble to data.frame
  	parsed <- as.data.frame(parsed, stringsAsFactors = FALSE)
  	attr(parsed, "spec") <- NULL
  	expect_equal(parsed, r_object)
  	})

• https://github.com/rstudio/plumber/blob/f4230ed45fa67d57179a901976245687427559d2/tests/testthat/test-serializer-feather.R

Thank you!

[r2evans]

Just missing tests for the serializer and parser

You can't see the uncommitted new-file tests/testthat/test-serializer-excel.R on my local computer? (Oops)

[r2evans]

@schloerke should be good now, tests are confirmed committed, devtools::test() is clean.

[schloerke]

Thank you!!