Special URL handling for DTML pages
===================================

When a DTML page containing a head tag is visited, without a trailing
slash, the base href isn't set.  When visited with a slash, it is:

  >>> print http(r"""
  ... POST /+/zope.app.dtmlpage.DTMLPage%3D HTTP/1.1
  ... Authorization: Basic mgr:mgrpw
  ... Content-Length: 497
  ... Content-Type: multipart/form-data; boundary=---------------------------824954900632940947241835499
  ... Referer: http://localhost:8081/+/zope.app.dtmlpage.DTMLPage%3D
  ... 
  ... -----------------------------824954900632940947241835499
  ... Content-Disposition: form-data; name="field.source"
  ... 
  ... 
  ... 
  ... 
  ... Eek
  ... 
  ... 
  ... 
  ... -----------------------------824954900632940947241835499
  ... Content-Disposition: form-data; name="UPDATE_SUBMIT"
  ... 
  ... Add
  ... -----------------------------824954900632940947241835499
  ... Content-Disposition: form-data; name="add_input_name"
  ... 
  ... dtml.html
  ... -----------------------------824954900632940947241835499--
  ... """)
  HTTP/1.1 303 See Other
  ...

  >>> print http(r"""
  ... GET /dtml.html HTTP/1.1
  ... Authorization: Basic mgr:mgrpw
  ... """)
  HTTP/1.1 200 Ok
  ...

  Eek

  <BLANKLINE>

  >>> print http(r"""
  ... GET /dtml.html/ HTTP/1.1
  ... Authorization: Basic mgr:mgrpw
  ... """)
  HTTP/1.1 200 Ok
  ...

  Eek

  <BLANKLINE>
