Fixed code added in 0c20b5c8013f93147c221f7c035f465c7825d10b
(PR #1505).

Tests added is #1663 pass with this patch.
updated the way the line color + alpha is handled to match how it is
done for markers.  If a 4-tuple is passed in, the alpha from that will
be used, with the kwarg alpha over-ridding it.
Updated the alpha/translucent test to also test lines and a mix of
setting alpha via color and via kwarg
Merge pull request #1695 from tacaswell/alpha_kwarg_fix

Alpha kwarg fix