Force catch_errors into pipeline

This commit adds a hook for WSGI applications
(e.g. proxy.server.Application) to modify their WSGI pipelines. This
is currently used by the proxy server to ensure that catch_errors is
present; if it is missing, it is inserted as the first middleware in
the pipeline.

This lets us write new, mandatory middlewares for Swift without
breaking existing deployments on upgrade.

Change-Id: Ibed0f2edb6f80c25be182b3d4544e6a67c5050ad
change the last-modified header value with valid one

the Last-Modified header in Response didn't have a suitable
value - an integer part of object's timestamp.
This leads that the the if-[un]modified-since header with the
value from last-modified is always earlier than timestamp
and results the content is always newer than value of these
conditional headers.
Patched code returns math.ceil() of object's timestamp
in Last-Modified header so the later conditional header works
correctly

Closes-Bug: #1248818
Change-Id: I1ece7d008551bf989da74d23f0ed6307c45c5436
Merge "change the last-modified header value with valid one"
Merge "Force catch_errors into pipeline"