allowed_attributes settings wipes out default allowed_attributes unexpectedly.

[tian-yan]

by default, allowed_attributes for <img> are ["src", "alt"].
When I configure my settings.py as below,

WAGTAILMARKDOWN = {
    "allowed_attributes": {"img": ["title"] 
                            },  
}

Based on the source code wagtailmarkdown/utils.py

if "allowed_attributes" in settings.WAGTAILMARKDOWN:
            bleach_kwargs["attributes"] = {
                **bleach_kwargs["attributes"],
                **settings.WAGTAILMARKDOWN["allowed_attributes"],
            }

Newly added allowed_attributes should be added on top of default allowed_attributes. However, based on my testing here, default allowed_attributes are wiped out and the resulted <img> only has attributetitle=" " now.

'<p><img title=" "></p>\n'
'<p><img title=" "></p>\n'

problem can be solved by including default allowed_attributes in the settings.py configurations.

WAGTAILMARKDOWN = {
    "allowed_attributes": {"img": ["title", "src", "alt"] 
                            },  
}

It is a minor issue, but since I experienced it, just share with the team.

[zerolab]

#108 aims to address this, and provide 2 additional settings to control how the allowed_*, as well as the extension* settings behave (default: extend. but can be replaced)