Treat directory objects as not found

Make StaticWeb search for an index file rather than returning a 0-byte
object.

Add new StaticWeb header to identify a directory marker object:
X-Container-Meta-Web-Directory-Type (default to application/directory)
so user can specify the content type for directory marker objects.
This required requesting the container headers earlier in the code and
clearing the value of _error for non-404 errors.

Add unit tests for directory object scenarios.
Fix end tags in an unrelated HTML block.

Bug 1178817

DocImpact

Change-Id: I561f00c099feaa82fd658f5050bd90c76717da24
Merge "Treat directory objects as not found"