Handle HTTP redirects properly

[lesserwhirls]

To be consistent with other services, TDS 5.0 serves out all catalogs using

thredds/catalog

The top level catalog's old url:

thredds/catalog.(html|xml)

redirects to:

thredds/catalog/catalog.(html|xml)

which siphon catches. However, the TDSCatalog object still has the old url, and thus any relative catalogRefs are constructed incorrectly (except for catalogRefs generated by catalogScan elements). For example, a TDS 5.0 catalogRef to forecastModels.xml is turned into an absolute URL of:

/thredds/forecastModels.xml

instead of the new, correct absolute URL of:

/thredds/catalog/forecastModels.xml

A request to the old absolute URL does not get a redirect, and thus fails with a 404.

Perhaps the correct thing to do here is check for the redirect to set the catalog objects "cat_url" attribute in TDSCatalog and make sure that absolute URLs for catalogRefs are built with the correct url. This should keep backwards compatibility with TDS <= 4.6 while future proofing for TDS 5.0.

[lesserwhirls]

The real issue is that we do not handle http redirects properly.