RDoc::RDoc#parse_files
(ruby/rdoc#1274)

Commit ruby/rdoc@6cf6e1647b97, which went to v6.5.0, changed `RDoc::Options#parse`
to not call `#finish` in it. While the commit adjusted other call sites,
it missed `lib/rdoc/rubygems_hook.rb`.

`RDoc::Options#finish` prepares the include paths for `:include:`
directives. This has to be done before starting to parse sources.

I think this should fix ruby/net-http#193 +
ruby/net-http#194.

ruby/rdoc@d62da8ca09

…1347)

Probably fixes #1343

`options.finish` is called twice while generating document through
rubygems_hook.
If gemspec has `spec.rdoc_options << '--ri'`, document generation fails.
Looks like this double-finish is introduced in #1274

### Detail of the bug
```ruby
options = ::RDoc::Options.new
options.parse args # if --ri is specified, generator is set to 'ri'
options.finish # @template='ri' and @template_dir=nil is configured here
@rdoc.parse_files options.files

document 'ri', options, @rdoc_dir
document 'darkfish', options, @rdoc_dir # Calls options.setup_generator('darkfish') and options.finish but @template_dir is already configured for 'ri'
```

### Fix
Constraints are:
- Need to call finish only once for each RDoc::Options instance
- `finish` should be called before `rdoc.parse_files(options.files)`
- `finish` should be called after `options.setup_generator`
- It is better to call `rdoc.parse_files` only once

We need to use a different RDoc::Options object for parse_files phase
and document generation phase.