refactor string_extend_chars: return when obj type is not string
refactor string_extend_chars: return when obj type is not string

aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust