not [a-zA-Z0-9+-.]) before there is a ":".

This is particularly helpful in the erroneous-but-relatively-common
case of relative URLs containing an unescaped colon in the query
string--see the added test for an example.

R=rsc
APPROVED=rsc
DELTA=15  (15 added, 0 deleted, 0 changed)
OCL=30354
CL=30356