Hi, and thank you for your answer.

However this is not strictly related to the newline, but also to some small idiosyncrasies and different behavior among py2 and py3 (and even in py2 using Header() or Charset()):

# py2.7, non-unicode str
>>> H('test', 'utf-8').encode()
'=?utf-8?q?test?='

>>> Charset('utf-8').header_encode('test')
'=?utf-8?q?test?='


# py2.7, unicode str
>>> H(u'test', 'utf-8').encode()   # this is the only different result
'test'

>>> Charset('utf-8').header_encode(u'test')
u'=?utf-8?q?test?='



# py3.4, unicode
>>> H('test', 'utf-8').encode()                                            
'=?utf-8?q?test?='                                                      
                
# py3.4, bytes                                                                
>>> H(b'test', 'utf-8').encode()                                             
'=?utf-8?q?test?='


As you can see, the only when using unicode strings in py2.7 no header encoding is done if the unicode string contains only ascii chars.