When filtering, do not consider declaration lines from other files.

Before this, our logic for line number filtering asked `RSpec.world`
for the preceding declaration line of a particular line number,
*without* passing it a file name, which meant that it looked at
the declaration line numbers from _all_ files. This was prone to
producing weird filtering behavior.

Fixes #2136.

Author: myronmarston

lib/rspec/core/world.rb

@@ -186,8 +186,10 @@ def descending_declaration_line_numbers_by_file
           declaration_locations = FlatMap.flat_map(example_groups, &:declaration_locations)
           hash_of_arrays = Hash.new { |h, k| h[k] = [] }
 
-          line_nums_by_file = declaration_locations.each_with_object(hash_of_arrays) do |(file_name, line_number), hash|
+          # TODO: change `inject` to `each_with_object` when we drop 1.8.7 support.
+          line_nums_by_file = declaration_locations.inject(hash_of_arrays) do |hash, (file_name, line_number)|
             hash[file_name] << line_number
+            hash
           end
 
           line_nums_by_file.each_value do |list|