string.c: respect the actual encoding

	* string.c (rb_enc_str_coderange): respect the actual encoding of
	  if a BOM presents, and scan for the actual code range.
	  [ruby-core:91662] [Bug #15635]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67181 b2dd03c8-39d4-4d8f-98ff-823fe69b080e