Populate req_hdrv with custom headers
Update tests to check for req_hdrv
Fix memory allocation bug

The current_size variable which holds the size in bytes of the rep_hdrv
char* array was only set once, even as members were added, so then the
size passed into reallocs was inaccurate.
Create function for custom header processing

Also improve comments and check return value for curl_slist_append()
calls.
Implement hdrv_append() function

Implement hdrv_append() which is a general function for appending to a
char* array. It is tailored for use with the req_t datatype in that it
is designed to be called like this:

hdrv_append(&(req->req_hdrv), &req->req_hdrc, somechar);

so that the *hdrc variable is also updated.

Also use it to add the "Content-Length: 0" header which was not included
previously.
Merge branch 'feature/req_headers' into develop