Use https:// protocol instead of git:// when a proxy is set

As per issue https://github.com/bower/bower/issues/731, resolving `git://` addresses to `https://` if a `proxy` or `httpsProxy` configuration is passed into resolvers. Change happens in `lib/core/resolvers/GitRemoteResolver.js` after after `GitResolver` [and `Resolver`] is mixed in, so it modifies only current 'working' copy of `source` [`this._source` rather than `decEndpoint.source`]. This allows `lib/core/resolvers/GitHubResolver.js` to still do the public/non-public check, since it now uses the `decEndpoint` metadata, rather than the resolved source.

This should help smooth out some difficulty with bower users who do not have `git` configured to use `https.insteadOf` to rewrite URLs.