merge revision(s) 39822,39856: [Backport #8141]

	* string.c (rb_str_conv_enc_opts): convert with one converter, instead
	  of re-creating converters for each buffer expansion.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@40256 b2dd03c8-39d4-4d8f-98ff-823fe69b080e