[wsgi] If multipart cannot parse the POST content, let it pass. multipart can only parse html form data. It the content_type is, for example, "application/json", get_posted_data should not fail but just stop trying to read the content. Closes #4421845

addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile