HTTP parser does not properly strip spaces and tabs from the beginnings of header values

The HTTP RFCs specify that header values may be optionally prefixed and/or suffixed with any number of spaces and/or tabs, and that these are to be ignored. Libevent implements this correctly for suffixes, but not for prefixes.

For example, if you send the following request to libevent:

GET / HTTP/1.1\r\n
Host: whatever\r\n
Test: \t abc \t \r\n
\r\n

...it sees a Test header value of \t abc, but it should see abc.