Serve MessageDialogs and exception views as text/html

[perrinjerome]

Now that Zope uses text/plain by default, MessageDialogs were served as text/plain. Keep compatibility by returning a special string with asHTML method, that ZPublisher.HTTPResponse.HTTPResponse.setBody understands.

MessageDialogs are deprecated and do not integrate well in Zope >= 4 ZMI, but they are used in some old products.

Fixes issues discussed in #1079 (comment)

[perrinjerome]

looks odd in the new ZMI, but it's better than

---

This implementation is a bit tricky, it uses

Zope/src/ZPublisher/HTTPResponse.py

Lines 541 to 544 in 6aa5e4d

	if hasattr(body, 'asHTML'):
	body = body.asHTML()
	if content_type is None:
	content_type = 'text/html'

the MessageDialogs are supposed to be returned directly ( see https://github.com/nakagami/Products.ZSQLiteDA/blob/c89c190f9572070607893ee09f627c3862370905/Products/ZSQLiteDA/DA.py#L128-L133 ), so at this level we don't have access to requests and response

/cc @dataflake @GiaBen

[perrinjerome]

the test on zope 4 seems to show a regression with this approach

[perrinjerome]

the test on zope 4 seems to show a regression with this approach

It's OK now.

For the details: in the previous version I was changing App.special_dtml.HTML to return html and this made this DTML Document with content index served as text/html instead of the expected text/plain

Zope/src/Testing/ZopeTestCase/zopedoctest/testFunctionalDocTest.py

Line 118 in 6c199cd

self.folder.addDTMLDocument('index_html', file='index')

[perrinjerome]

Thanks @dataflake for the amendments 👍