fix auto-indent after multiline string

Author: tompng

lib/irb/ruby-lex.rb

@@ -182,6 +182,7 @@ def find_prev_spaces(line_index)
         if line_count >= line_index
           return prev_spaces
         end
+        next if t.event == :on_tstring_content || t.event == :on_words_sep
         if (@tokens.size - 1) > i
           md = @tokens[i + 1].tok.match(/(\A +)/)
           prev_spaces = md.nil? ? 0 : md[1].count(' ')