Fix two bugs in #expect_correction

- When an attempt at correcting does not correct anything, just end the
  loop.
- Reset the memoized `@token_table` on each iteration. It would be
  cleaner to use a new `cop` instance on each iteration, but I am not
  sure how to fix that.

lib/rubocop/rspec/expect_offense.rb

@@ -128,13 +128,16 @@ def expect_correction(correction, loop: false)
 
           break corrected_source unless loop
           break corrected_source if cop.corrections.empty?
+          break corrected_source if corrected_source == @processed_source.buffer.source
 
           if iteration > RuboCop::Runner::MAX_ITERATIONS
             raise RuboCop::Runner::InfiniteCorrectionLoop.new(@processed_source.path, [])
           end
 
           # Prepare for next loop
           cop.instance_variable_set(:@corrections, [])
+          # Cache invalidation. This is bad!
+          cop.instance_variable_set(:@token_table, nil)
           @processed_source = parse_source(corrected_source,
                                            @processed_source.path)
           _investigate(cop, @processed_source)