Add spec for require after $LOAD_PATH change

When developer add something to $LOAD_PATH and require once again the
same file ruby should not require it once again.

Example:

Rubinius have lib/racc/parser.rb file. And this file will require after
bootstrap.

  require 'racc/parser'

After require 'rubygems' $LOAD_PATH will change and now we have access
to two version on 'racc/parser' (from Rubinius /lib and /gems/racc).
When user try to require once again rubinius will search via $LOAD_PATH
and hit racc/parser from _RUBYGEMS_. In ruby 1.9 mode all paths are
expanded so rubinius will require once again 'racc/parser'. This is
because in $LOADED_FEATURES we can find only
"[rubinius_path]/lib/racc/parser.rb". When CodeLoader will find file in
$LOAD_PATH ([rubygems]/racc/parser.rb) and full path is not part of
$LOADED_FEATURES this file will be load once again.
Add a spec for String#[]= [regexp, index]

When the index points to unmatched captures.