HTTP_ACCEPT_CHARSET parsing fragile

Received a malformed HTTP_ACCEPT_CHARSET which raised a ValueError instead
of trying its best. The header was from a bot, which I reported the issue
to. But seems like Zope could be a bit more resilient when dealing with a
broken HTTP_ACCEPT_CHARSET.

Here is the malformed header.

 'HTTP_ACCEPT_CHARSET': 'utf-8;q=0.7,iso-8859-1;q=0.2*;q=0.1',

The issue is a missing comma in that last bit, between the '0.2' and the
asterisk (*).

Below is the full traceback from Zope-2.10.5. I checked and the issue
remains in 2.11.1, though the lines numbers are a bit different.

Error Type: ValueError
Error Value: too many values to unpack

Traceback (innermost last):
    * Module ZPublisher.Publish, line 119, in publish
    * Module ZPublisher.mapply, line 88, in mapply
    * Module ZPublisher.Publish, line 42, in call_object
    * Module OFS.DTMLMethod, line 144, in __call__
      <DTMLMethod at /www_org/ContentRoot/index_html used for /www_org/ContentRoot/visitors/tell_a_friend>
      URL: http://www.mengonline.com/index_html/manage_main [^]
      Physical Path:/www_org/ContentRoot/index_html
    * Module DocumentTemplate.DT_String, line 476, in __call__
    * Module Products.DotOrg.Pages.KContent, line 237, in __call__
    * Module Products.DotOrg.KOFS.DTMLMethod, line 34, in __call__
    * Module Products.DotOrg.KOFS.DTMLUnicode, line 40, in __call__
    * Module OFS.DTMLMethod, line 144, in __call__
      <KContent at /www_org/ContentRoot/visitors/tell_a_friend/content>
      URL: http://www.mengonline.com/visitors/tell_a_friend/content/manage_main
      Physical Path:/www_org/ContentRoot/visitors/tell_a_friend/content
    * Module DocumentTemplate.DT_String, line 476, in __call__
    * Module Shared.DC.Scripts.Bindings, line 327, in __render_with_namespace__
    * Module Shared.DC.Scripts.Bindings, line 350, in _bindAndExec
    * Module App.special_dtml, line 178, in _exec
    * Module DocumentTemplate.DT_Let, line 76, in render
    * Module DocumentTemplate.ustr, line 44, in ustr
    * Module Products.DotOrg.Forms.KAppForm, line 87, in __str__
    * Module Products.DotOrg.Forms.KForm, line 326, in __str__
    * Module Products.DotOrg.Forms.KForm, line 274, in ordered_layout
    * Module Shared.DC.Scripts.Bindings, line 313, in __call__
    * Module Shared.DC.Scripts.Bindings, line 350, in _bindAndExec
    * Module Products.PageTemplates.PageTemplateFile, line 129, in _exec
    * Module Products.PageTemplates.PageTemplate, line 89, in pt_render
    * Module zope.pagetemplate.pagetemplate, line 117, in pt_render
    * Module zope.tal.talinterpreter, line 271, in __call__
    * Module zope.tal.talinterpreter, line 346, in interpret
    * Module zope.tal.talinterpreter, line 408, in do_startTag
    * Module zope.tal.talinterpreter, line 485, in attrAction_tal
    * Module Products.PageTemplates.Expressions, line 229, in evaluateText
    * Module Products.PageTemplates.Expressions, line 255, in _handleText
    * Module Products.PageTemplates.unicodeconflictresolver, line 69, in resolve
    * Module zope.publisher.http, line 1001, in getPreferredCharsets
ValueError: too many values to unpack