[SPARK-10219] [SPARKR] Fix varargsToEnv and add test case

cc sun-rui davies

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes apache#8475 from shivaram/varargs-fix.

R/pkg/R/utils.R

@@ -314,7 +314,8 @@ convertEnvsToList <- function(keys, vals) {
 
 # Utility function to capture the varargs into environment object
 varargsToEnv <- function(...) {
-  pairs <- as.list(substitute(list(...)))[-1L]
+  # Based on http://stackoverflow.com/a/3057419/4577954
+  pairs <- list(...)
   env <- new.env()
   for (name in names(pairs)) {
     env[[name]] <- pairs[[name]]

R/pkg/inst/tests/test_sparkSQL.R

@@ -1060,6 +1060,12 @@ test_that("parquetFile works with multiple input paths", {
   parquetDF <- parquetFile(sqlContext, parquetPath, parquetPath2)
   expect_is(parquetDF, "DataFrame")
   expect_equal(count(parquetDF), count(df) * 2)
+
+  # Test if varargs works with variables
+  saveMode <- "overwrite"
+  mergeSchema <- "true"
+  parquetPath3 <- tempfile(pattern = "parquetPath3", fileext = ".parquet")
+  write.df(df, parquetPath2, "parquet", mode = saveMode, mergeSchema = mergeSchema)
 })
 
 test_that("describe() and summarize() on a DataFrame", {