Proxy authorization fixes

Proxy authorization fixes

* HTTP proxy authentication failed because EncodeBase64() adds a trailing newline
  which corrupts the HTTP headers.

* HTTP proxies use the "Proxy-Authorization" header for authenticating to the proxy.
  The "Authorization" header is required to be forwarded to the destination.
  (See https://tools.ietf.org/html/rfc7235#section-4.2)

* If a SOCKS5 server was configured to accept connections both with and without
  authentication, there was no way to connect to it without a username/password pair.
  That's because we always reported we could do username/password authentication and
  the proxy - obviously - selected the most secure method: username/password.

  Now if the "Enable authentication" checkbox is cleared we won't offer any
  authentication methods (except for NO_AUTH_REQUIRED, which isn't actually an
  authentication method). If the checkbox is checked, we offer all known methods
  (currently username/password only) which will require valid credentials on all
  proxies supporting basic authentication. But if a proxy accepts only
  unauthenticated connections we are still able to connect to it.

* Though a SOCKS4/4a server doesn't offer any real authentication and the
  "Username" field of the request is optional, we send a username only if
  the "Enable authentication" checkbox is checked.