Support URI reserved characters in proxy server passwords.

[petelacey]

I was the guy who submitted the original PR for proxy server support in Rollbar. Unfortunately, that code will fail in the proxy requires authentication, the credentials include URI-reserved characters, such as '@' or '?', and the credentials are provided as part of the Rollbar configuration.

This PR addresses that by holding the proxy configuration information in a Struct instead of a URI object. This PR includes tests, but the fix has also been tested in a runtime environment.

[rokob]

This is an interesting case, Net::HTTP is just fine with proxy passwords with special characters but URI is not. I just double checked that sending a request with Net::HTTP after setting the proxy password to something with an @ works just fine. URI#password= on the other hand throws a URI::InvalidComponentError: bad component(expected user component). This seems like a reasonable approach to me because we never use the returned URI object as anything but a bag of properties.