fixes #605: send DT requests via POST instead of the default GET

I have tested it with a few examples, and I did not see problems.

BTW, URLencode() and URLdecode() are pretty slow in base R, and I'm wondering if anyone is interested in rewriting them in C or C++. It should be very simple to rewrite them, but I have little experience in C/C++. The slowness should rarely matter, though, since the URL's to be processed are normally relatively short. I tested a character string of 200K, and URLdecode() can take 40 seconds to decode it.