When trying to encode an email header with a newline in it, correct encoding is done only for strings and not for unicode strings.
In fact, for unicode strings, encoding is only done if a non ascii character is contained in it.

The attached patch should fix the problem.

Simple example to reproduce the problem:
>>> from email.Header import Header as H

# correctly encoded
>>> H('two\r\nlines', 'utf-8').encode()
'=?utf-8?q?two=0D=0Alines?='

# unicode string not encoded
>>> H(u'two\r\nlines', 'utf-8').encode()
'two\r\nlines'

# unicode string with non ascii chars, correctly encoded
>>> H(u'two\r\nlines and \xe0', 'utf-8').encode()
'=?utf-8?b?dHdvDQpsaW5lcyBhbmQgw6A=?='