handle complete responses to https range requests

handle complete responses to https range requests

Servers might respond with a complete file either because they don't
support Ranges at all or the If-Range condition isn't statisfied, so we
have to parse the headers curl gets ourself to seek or truncate the file
we have so far.

This also finially adds the testcase testing a bunch of partial
situations for both, http and https - which is now all green.

Closes: 617643, 667699
LP: 1157943