[SPARK-13905][SPARKR] Change signature of as.data.frame() to be consistent with the R base package.

## What changes were proposed in this pull request?

Change the signature of as.data.frame() to be consistent with that in the R base package to meet R user's convention.

## How was this patch tested?
dev/lint-r
SparkR unit tests

Author: Sun Rui <rui.sun@intel.com>

Closes #11811 from sun-rui/SPARK-13905.

R/pkg/R/DataFrame.R

@@ -2296,12 +2296,8 @@ setMethod("fillna",
 #' }
 setMethod("as.data.frame",
           signature(x = "DataFrame"),
-          function(x, ...) {
-            # Check if additional parameters have been passed
-            if (length(list(...)) > 0) {
-              stop(paste("Unused argument(s): ", paste(list(...), collapse = ", ")))
-            }
-            collect(x)
+          function(x, row.names = NULL, optional = FALSE, ...) {
+            as.data.frame(collect(x), row.names, optional, ...)
           })
 
 #' The specified DataFrame is attached to the R search path. This means that

R/pkg/R/generics.R

@@ -397,7 +397,10 @@ setGeneric("arrange", function(x, col, ...) { standardGeneric("arrange") })
 
 #' @rdname as.data.frame
 #' @export
-setGeneric("as.data.frame")
+setGeneric("as.data.frame",
+           function(x, row.names = NULL, optional = FALSE, ...) {
+             standardGeneric("as.data.frame")
+           })
 
 #' @rdname attach
 #' @export

R/pkg/inst/tests/testthat/test_context.R

@@ -26,7 +26,7 @@ test_that("Check masked functions", {
   maskedBySparkR <- masked[funcSparkROrEmpty]
   namesOfMasked <- c("describe", "cov", "filter", "lag", "na.omit", "predict", "sd", "var",
                      "colnames", "colnames<-", "intersect", "rank", "rbind", "sample", "subset",
-                     "summary", "transform", "drop", "window")
+                     "summary", "transform", "drop", "window", "as.data.frame")
   expect_equal(length(maskedBySparkR), length(namesOfMasked))
   expect_equal(sort(maskedBySparkR), sort(namesOfMasked))
   # above are those reported as masked when `library(SparkR)`

R/pkg/inst/tests/testthat/test_sparkSQL.R

@@ -1863,6 +1863,9 @@ test_that("Method as.data.frame as a synonym for collect()", {
   expect_equal(as.data.frame(irisDF), collect(irisDF))
   irisDF2 <- irisDF[irisDF$Species == "setosa", ]
   expect_equal(as.data.frame(irisDF2), collect(irisDF2))
+
+  # Make sure as.data.frame in the R base package is not covered
+  expect_that(as.data.frame(c(1, 2)), not(throws_error()))
 })
 
 test_that("attach() on a DataFrame", {