Add a "relaxed" variant that accepts one case of broken grammar.

Also add better error handling, and tests for strict and relaxed
behaviour.

rfc6266.py

@@ -166,7 +166,7 @@ def ensure_charset(text, encoding):
         return text
 
 
-def parse_headers(content_disposition, location=None):
+def parse_headers(content_disposition, location=None, relaxed=False):
     """Build a ContentDisposition from header values.
     """
 
@@ -201,25 +201,34 @@ def parse_headers(content_disposition, location=None):
     # of LWS.
     assert is_lws_safe(content_disposition)
 
-    parsed = content_disposition_value.parse(content_disposition)
+    if relaxed:
+        parser = content_disposition_value_relaxed
+    else:
+        parser = content_disposition_value
+
+    try:
+        parsed = parser.parse(content_disposition)
+    except FullFirstMatchException:
+        return ContentDisposition(location=location)
     return ContentDisposition(
         disposition=parsed[0], assocs=parsed[1:], location=location)
 
 
-def parse_httplib2_response(response):
+def parse_httplib2_response(response, **kwargs):
     """Build a ContentDisposition from an httplib2 response.
     """
 
     return parse_headers(
-        response.get('content-disposition'), response['content-location'])
+        response.get('content-disposition'),
+        response['content-location'], **kwargs)
 
 
-def parse_requests_response(response):
+def parse_requests_response(response, **kwargs):
     """Build a ContentDisposition from a requests (PyPI) response.
     """
 
     return parse_headers(
-        response.headers.get('content-disposition'), response.url)
+        response.headers.get('content-disposition'), response.url, **kwargs)
 
 
 def parse_ext_value(val):
@@ -318,6 +327,12 @@ with DroppedSpace():
     content_disposition_value = (
         disposition_type & Star(Drop(';') & disposition_parm))
 
+    # Allows nonconformant final semicolon
+    # I've seen it in the wild, and browsers accept it
+    # http://greenbytes.de/tech/tc2231/#attwithasciifilenamenqs
+    content_disposition_value_relaxed = (
+        content_disposition_value & Optional(Drop(';')))
+
 
 def is_token_char(ch):
     # Must be ascii, and neither a control char nor a separator char

test_rfc6266.py

@@ -48,6 +48,19 @@ def test_location_fallback():
     ).filename_unsafe == u'baré.py'
 
 
+def test_strict():
+    # Trailing ; means the header is rejected
+    assert parse_headers('attachment;').disposition == 'inline'
+    assert parse_headers('attachment; key=val;').disposition == 'inline'
+
+
+def test_relaxed():
+    assert parse_headers(
+        'attachment;', relaxed=True).disposition == 'attachment'
+    assert parse_headers(
+        'attachment; key=val;', relaxed=True).disposition == 'attachment'
+
+
 def test_roundtrip():
     def roundtrip(filename):
         return parse_headers(build_header(filename)).filename_unsafe