rb_str_freeze may resize oversized buffers to save memory, so favor
it over rb_obj_freeze. This is useful because IO methods do not
prematurely shrink buffers, as they are likely to be overwritten
with full data.
* string.c (Init_String): use rb_str_freeze for String#freeze
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51170 b2dd03c8-39d4-4d8f-98ff-823fe69b080e