Dont wrap IEW with spans when they are in fosterable posns in tables

* On the following wikitext snippet, Parsoid was adding span wrappers
  around IEW emitted by the {{WDL}} tpl. These would then get fostered
  out and introduce dirty diffs by emitting <span></span> on RTing.
----
{|
|5th {{WDL|7|2|2|3}} || 5 || 7 || 6
|}
----

* This patch makes fixes by not span-wrapping IEW in fosterable posns
  in tables.

* Also fixed getAboutSiblings to continue collecting siblings across
  IEW gaps when in fosterable position.

* Fixes parsing of above snippet.

* No change in parser test results (TODO: need to come up with a good
  test case for this to prevent regressions).

Change-Id: Idaf069866510f63921334c272982ec6e38297bea