I encountered a problem with replacing the 'Subject' header of an email. After serializing it again, the utf8 encoding was wrong. It seems to be occurring when folding the internal header objects.

Example:
>> email.policy.default.fold_binary('Subject', email.policy.default.header_store_parse('Subject', 'Hello Wörld! Hello Wörld! Hello Wörld! Hello Wörld!Hello Wörld!')[1])
Expected output: b'Subject: Hello =?utf-8?q?W=C3=B6rld!_Hello_W=C3=B6rld!_Hello_W=C3=B6rld!?=\n Hello =?utf-8?q?W=C3=B6rld!Hello_W=C3=B6rld!?=\n' (or similar)
Actual output: b'Subject: Hello =?utf-8?q?W=C3=B6rld!_Hello_W=C3=B6rld!_Hello_W=C3=B6rld!?=\n Hello =?utf-8?=?utf-8?q?q=3FW=3DC3=3DB6rld!Hello=3F=3D_W=C3=B6rld!?=\n'

I'm running Python 3.7.3 on Arch Linux using Linux 5.0.