Fix warning under older versions of JRuby that didn't provide $-w

The warning was: "warning: global variable `$-w' not initialized"
Raise a more appropriate exception when given an unknown response object

The change in responder.rb should be backwards-compatible (although it seems
unlikely that anyone's depending on it) because ArgumentError < StandardError.
The change in net_http.rb won't affect anyone because it's an internal sanity
check -- the guard in responder.rb catches everything first.

Also, a little related cleanup and test coverage.
Let original method process its arguments so we don't have to keep up with changes

For example, there's now a @continue_timeout that we haven't been setting.