Fix another issue with coverage merging

Previously, only the first command_name => coverage_data pair of each
result set file was getting merged.

lib/simplecov.rb

@@ -79,8 +79,10 @@ def collate(result_filenames, profile = nil, &block)
 
       results = result_filenames.map do |filename|
         # Re-create each included instance of SimpleCov::Result from the stored run data.
-        SimpleCov::Result.from_hash(JSON.parse(File.read(filename)) || {})
-      end
+        (JSON.parse(File.read(filename)) || {}).map do |command_name, coverage|
+          SimpleCov::Result.from_hash(command_name => coverage)
+        end
+      end.flatten
 
       # Use the ResultMerger to produce a single, merged result, ready to use.
       @result = SimpleCov::ResultMerger.merge_results(*results)