This could happen if a sequence of '0' parts was followed by a longer
sequence of '0' parts at the end of the host. The first sequence was
being used for the compress, and not the second.
For example, [1:1:0:0:1:0:0:0] was being serialized as: [1:1::1:0:0:0]
instead of [1:1:0:0:1::].
Fix this by checking at the end of the loop if we are in the middle of a
sequence of '0' parts that is longer than the current longest.
This is defined in the spec, but was missing in our table. Fix this, and
add a spec comment for what is missing. Also begin a basic text based
test for URL, so we can get some coverage of LibWeb's usage of URL too.