Make .rdoc_options file plain ruby objects only

[nobu]

Currently, the .rdoc_options file generated by --write-options is not loadable.

$ rm -f .rdoc_options 

$ unset RDOCOPT

$ ruby -I./lib exe/rdoc --write-options

$ grep ^exclude: .rdoc_options 
exclude: !ruby/regexp /~\z|\.orig\z|\.rej\z|\.bak\z|\.gemspec\z/

$ ruby -I./lib exe/rdoc
uh-oh! RDoc had a problem:
Tried to load unspecified class: Regexp

run with --debug for full backtrace