http: ignore Version: header if explicitly set by client

http: ignore Version: header if explicitly set by client

When Unicorn receives a request with a "Version" header, the
HttpParser transforms it into "HTTP_VERSION". After that tries to add
it to the request hash which already contains a "HTTP_VERSION" key
with the actual http version of the request. So it tries to append the
new value separated by a comma. But since the http version is a
freezed constant, the TypeError exception is raised.

According to the HTTP RFC
(http://www.w3.org/Protocols/rfc2616/rfc2616-sec7.html#sec7.1) a
"Version" header is valid. However, it's not supported in rack, since
rack has a HTTP_VERSION env variable for the http version. So I think
the easiest way to deal with this problem is to just ignore the header
since it is extremely unusual. We were getting it from a crappy bot.

ref: http://mid.gmane.org/AANLkTimuGgcwNAMcVZdViFWdF-UcW_RGyZAue7phUXps@mail.gmail.com

Acked-by: Eric Wong <normalperson@yhbt.net>