add helper.fromAsString(node, sender) to be complete.

I also would like to have a more complex test case aka:

toplevel (toplvel@)
-> sub -> toplevel@
->an attached rfc822 mail with own sender -> second@
   -> subsub -> second@

and for all 4 parts make sure that th correct from address is returned and for all extra parts, that must return the correct value.

That actually would not be compatible with the current behavior and is beyond the scope of this change (IMO). NodeHelper::fromAsString() returns sender of the top-level message, which always is a KMime::Content with a null parent. Even for an encapsulated message this function always returns a sender of the top-level message.

I can add X_FAIL test cases so we know this should be fixed/solved, but the correct detection of nested sub-message may be non-trivial and might break some other code that might rely on the current behaviour.