fix(curl): Do not loop the same data when sending with curl

[jan-auer]

Good bug.

The curl transport implementation works like this:

• Serialize the event into an in-memory buffer
• Determine the length of the buffer and let curl read it
• In the read function, pass a chunk into the buffer

However, the read function always sliced the beginning of the buffer, rather than reading it chunk by chunk. This is fixed by wrapping the buffer into a cursor, which implements read correctly.