Fix result collation

Previously, collation was being made using `Hash#merge`, and the
`ResultMerger` was being passed an already (incorrectly) merged result.

lib/simplecov.rb

@@ -77,9 +77,9 @@ def collate(result_filenames, profile = nil, &block)
       self.running = true
       self.pid = Process.pid
 
-      results = result_filenames.map { |filename| JSON.parse(File.read(filename)) || {} }.reduce(&:merge).map do |command_name, data|
+      results = result_filenames.map do |filename|
         # Re-create each included instance of SimpleCov::Result from the stored run data.
-        SimpleCov::Result.from_hash(command_name => data)
+        SimpleCov::Result.from_hash(JSON.parse(File.read(filename)) || {})
       end
 
       # Use the ResultMerger to produce a single, merged result, ready to use.