+        if bad_header_value_re.search(_value):
+            error_str = "Bad header value: {0!r} (bad char: {1!r})"
+            raise AssertionError(error_str.format(
+                _value, bad_header_value_re.search(_value).group(0)))

Why do you search the character twice? You can do something like:

match = bad_header_value_re.search(_value)
if match is not None:
  ... match..group(0) ...

Why do you only check value? You should also check _params:

parts = "; ".join(parts)
match = bad_header_value_re.search(parts)
...

And you should also check the name.

Should we do the same checks in httplib?