Fix retried requests (ATV-660)

When first creating a request, this.options.data is converted to a string (if it's not already a string).  That string is then converted to a buffer and transmitted as the request body.
When retrying a request, that buffer was stringified, then this string was converted to another buffer.  The retry request body then looks bogus (like '0=123&1=34&2=105&3=100&4=34' ...)
The content-type header was also trounced with the value 'application/x-www-form-urlencoded'.

This fix puts the Buffer data in a different variable and prevents the data from being re-stringified and re-bufferified when when retry() is called.