Encode URLs about rcites

[KevCaz]

Depending on this I may or may not use utils::encode() ropensci/vcr#240

[KevCaz]

There are several good options, can't say they're all as good, but I don't see any major drawback for any of those:

1. using modify_url() from httr
2. using curl::curl_escape() which is used at some point by httr::modify_url()
3. using utils::encode() which I do.

None of them will actually add the dependencies list because the 3 packages area already direct or indirect depencies.

[KevCaz]

This has been fixed in vcr see ropensci/vcr#247.