[Rd] encoding argument of source() in 3.5.0

[Stephen Berman]

In R 3.5.0 using the `encoding' argument of source() prevents loading
files from the internet; without the `encoding' argument files can be
loaded from the internet, but if they contain non-ascii characters,
these are not correctly displayed under MS-Windows (but they are
correctly displayed under GNU/Linux).  With R 3.4.{2,3,4} there is no
such problem: using `encoding' the files are loaded and non-ascii
characters are correctly displayed under MS-Windows (but not without
`encoding').  Here is a transcript from R 3.5.0 under GNU/Linux (the
URLs are real, in case anyone wants to try and reproduce the problem):

> ls()
character(0)
> source("http://home.versanet.de/~s-berman/source1.R", encoding="UTF-8")
> ls()
character(0)
> source("http://home.versanet.de/~s-berman/source2.R", encoding="UTF-8")
> ls()
character(0)
> source("http://home.versanet.de/~s-berman/source1.R")
> ls()
[1] "source.test1"
> source("http://home.versanet.de/~s-berman/source2.R")
> ls()
[1] "source.test1" "source.test2"
> source.test1()
[1] "This is a test."
> source.test2()
[1] "Non-ascii: äöüß"

(The four non-ascii characters are Unicode 0xE4, 0xF6, 0xFC, 0xDF.)
With 3.5.0 under MS-Windows, the transcript is the same except for the
display of the last output, which is this:

[1] "Non-ascii: äöüß"

(Here there are eight non-ascii characters, which display the Unicode
decompositions of the four non-ascii characters above.)

Here is a transcript from R 3.4.3 under MS-Windows (under GNU/Linux it's
the same except that the non-ascii characters are also correctly
displayed even without the `encoding' argument):

> ls()
character(0)
> source("http://home.versanet.de/~s-berman/source1.R")
> ls()
[1] "source.test1"
> source("http://home.versanet.de/~s-berman/source2.R")
> ls()
[1] "source.test1" "source.test2"
> source.test1()
[1] "This is a test."
> source.test2()
[1] "Non-ascii: äöüß"
> rm(source.test2)
> ls()
[1] "source.test1"
> source("http://home.versanet.de/~s-berman/source2.R", encoding="UTF-8")
> ls()
[1] "source.test1" "source.test2"
> source.test2()
[1] "Non-ascii: äöüß"

I did a web search but didn't find any reports of this issue, nor did I
see any relevant entry in the 3.5.0 NEWS, so this looks like a bug, but
maybe I've overlooked something.  I'd be grateful for any enlightenment.

Steve Berman