In the constructor values already is normalized (i.e. with underscores),
while kwargs contains items that are not normalized (i.e. with dashes).
Joining the dicts resulted in two entries that only differed by dash or
underscores. The setattr then set the value that occurred later in
items, thereby sometimes overriding the correct value with the default
one.

The get_raw_docs method accesses header field content-transfer-encoding
using the string 'content-transfer-type' so the raw doc dict always ends
up with that value set to empty string.

This test was not updated for a while and it doesn't make sense
in this context, when we create acceptance tests we can make sure
we cover this feature

The issue for acceptance tests is:
https://leap.se/code/issues/7341

Functionality that is not implemented should not fail, instead
it should be skipped until it is implemented.

Added a return to test_imap_store_fetch setup to make sure the deferred
is being handled

The lambdas take two args, so it needs to be a tuple.
Furthermore filter needs a collection.