Strip newlines from mulitline headers

As per the RFC https://tools.ietf.org/html/rfc3028

> Folding of long header lines (as described in [IMAIL] 3.4.8) is
> removed prior to interpretation of the data.  The folding syntax (the
> CRLF that ends a line plus any leading whitespace at the beginning of
> the next line that indicates folding) are interpreted as if they were
> a single space.

Author: russell

sifter/tests/header.py

@@ -1,9 +1,13 @@
+import re
 import sifter.grammar
 import sifter.grammar.string
 import sifter.validators
 
 __all__ = ('TestHeader',)
 
+# Newline striping
+_newline_re = re.compile(r'\n+\s+')
+
 # section 5.7
 class TestHeader(sifter.grammar.Test):
 
@@ -35,6 +39,7 @@ def evaluate(self, message, state):
         for header in self.headers:
             header = sifter.grammar.string.expand_variables(header, state)
             for value in message.get_all(header, []):
+                value = _newline_re.sub(" ", value)
                 for key in self.keylist:
                     key = sifter.grammar.string.expand_variables(key, state)
                     if sifter.grammar.string.compare(str(value), key, state,