400 Bad Request - Invalid Headers

There's a problem with Goutte HTTP header naming when it uppercase only the first character of the header name, for instance, User-agent. Guzzle sets its own user agent header as "User-Agent" with both initial letters uppercased. When a request is made, instead of one header overwriting the other, both are snet to the web server. IIS, for instance, rejects the request: 400 Bad Request - Invalid Headers.

This one line fix makes the first character of all parts of the header name uppercased.