disable RCurl:::mapUnicodeEscapes to get contents as-is

This change is a workaround for https://github.com/omegahat/RCurl/issues/1.

OAuth#OAuthRequest shouldn't change the response body.
Here is a sample code to explain this issue.

    > twitteR:::twInterfaceObj$doAPICall(url="http://abicky.net/hatena/rcurl/a.json")
    Error in twFromJSON(out) : 
      Error: Malformed response from server, was not JSON.
    The most likely cause of this error is Twitter returning a character which
    can't be properly parsed by R. Generally the only remedy is to wait long
    enough for the offending character to disappear from searches (e.g. if
    using searchTwitter()).

The response JSON is broken because OAuth#OAuthRequest change the JSON in this sample.


After applying this change, twitteR:::twInterfaceObj works correctly.

    > twitteR:::twInterfaceObj$doAPICall(url="http://abicky.net/hatena/rcurl/a.json")
    $a
    [1] "\\\"0\\\""