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email.header.Header class has its unicode representation which is slightly different from str representation.
>>> h = email.Header.make_header((('[XXX]', 'us-ascii'), ('Re:', 'us-ascii')))
>>> unicode(h)
u'[XXX]Re:'
>>> str(h)
'[XXX] Re:'
Note that a white space between ']' and 'R' is absent from unicode representation. A word-separating space should be needed in following contexts:
lastcs \ nextcs | ascii | other |
 ascii | sp | sp |
 other | sp | nosp |
Current code fails to put a space for ascii\ascii case.
Also, if the ascii string has a trailing space, it may result in a extraneous double space which has semantically equivalent to a single space and sometimes annoying. You should use qp encoding for ascii charset if you insist on the presence of the space.
The patch includes test code. :)