Duplicate options' hash

Duplication of options' hash in gem specification allows to do this in Gemfile:

```Gemfile
options_hash = {github: 'rails', branch: 'master'}
gem 'activemodel', options_hash
gem 'activerecord', options_hash
```

without damaging `options_hash` after passing it to `gem` method.

Without this fix, you can't pass the `options_hash` twice, because it will include 'env' and 'source' keys in it, and `gem` method [will complain about invalid keys](https://github.com/carlhuda/bundler/blob/master/lib/bundler/dsl.rb#L203).