Fixed in revision 83521 (py3k) and 83522 (release31-maint).
I made slight modifications to the patch to include non-iterable values,like int, for default too.

If you feel the documentation could be made better, please suggest the wordings for the sentence. I am leaving the bug open for this, if the current one is fine, we can close the bug.

David (RDM): I am not sure if we need a Deprecation Warning here.
If you actually see getheader() is supposed to return a string only as Headers are strings. I don't think someone should be consciously passing a list of items and excepting the list of items back.

', '.join of header values seem to be for situations in which get_all returns list of values, which is possible when there are different values for same header key.  This is a correct behavior IMO. The 2.x one is a mistake where it returned only the first value as a string.