Fixed headers parsing in CurlHttpClient

1. substr(pos, pos_end) was used instead of substr(pos, pos_end - pos)
2. CURLOPT_HEADERFUNCTION is called for single header so no need to
   merge and split them later

Change-Id: Iad852212258f870c2fe0683b0021ae99a020d12b
Reviewed-on: https://weave-review.googlesource.com/1562
Reviewed-by: Vitaly Buka <vitalybuka@google.com>