Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

shr.el (shr-render-td): Render td content with shr-descend, so style will be applied to <td> too.
 (shr-colorize-region): Colorize region even if we only have a background.
 (shr-tag-body): Fix color and background color inheritance.
 (shr-tag-body): Do not recolorize after shr-generic.
 (shr-tag-font): Let shr-generic colorize via inheritance.
shr.el (shr-descend): Only colorise something if we have a node that sets colors.
gnus-int.el (gnus-request-thread): Rework to take a header instead of a Message-ID to avoid having nnimap depend on gnus-sum.
shr.el (shr-tag-font): Colorize the region.
 (shr-tag-body): Ditto.
 (shr-tag-font): Actually let the styles be inherited instead of overwriting them.
 (shr-tag-font): Get the background color right.
 (shr-tag-style): Ignore all <style> tags for the moment.