Simple header validation fails with folded value

[kynx]

If the header value is folded and the continuation line contains leading whitespace:

Subject: Subject Subject Subject Subject Subject Subject Subject Subject Subject
 Subject

the simple header canonicalization strips the leading whitespace on the continuation line:

Subject: Subject Subject Subject Subject Subject Subject Subject Subject Subject
Subject 

This leads to an (incorrect) failed validation. According to https://www.rfc-editor.org/rfc/rfc6376#section-3.4.5 the leading whitespace on the continuation line should be preserved.