Get all of the headers added to a response.
Invoke the iterator callback for each header in the response,
using iterator_cls as first argument. Return number of entries
iterated over. iterator can be
NULL: in this case the function
just counts headers.
iterator should not modify the its key and value arguments, unless we know what we are doing.
Find and return a pointer to the value of a particular header from the
response. key must reference a zero-terminated string
representing the header to look for. The search is case sensitive.
NULL if header does not exist or key is
We should not modify the value, unless we know what we are doing.