Merge pull request #32 from tmtm/fix-read-create-many-objects

FIX: IO#read create a large number of objects.
Revert "FIX: IO#read create a large number of objects."
Merge pull request #33 from iij/revert-32-fix-read-create-many-objects

Revert "FIX: IO#read create a large number of objects."
for environment without mruby-string-ext.