Do not dup nonfrozen string on download (#754)

Fixes memory leak on download

Author: TheSmartnik

lib/httparty/text_encoder.rb

@@ -5,7 +5,7 @@ class TextEncoder
     attr_reader :text, :content_type, :assume_utf16_is_big_endian
 
     def initialize(text, assume_utf16_is_big_endian: true, content_type: nil)
-      @text = text.dup
+      @text = +text
       @content_type = content_type
       @assume_utf16_is_big_endian = assume_utf16_is_big_endian
     end