Don't read non .rb file as a spec

To avoid reading `core` file on reading core/ specs.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58713 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/mspec/lib/mspec/utils/script.rb

@@ -185,7 +185,7 @@ class MSpecScript
 
     patterns.each do |pattern|
       expanded = File.expand_path(pattern)
-      if File.file?(expanded)
+      if File.file?(expanded) && expanded.end_with?('.rb')
         return [expanded]
       elsif File.directory?(expanded)
         return Dir["#{expanded}/**/*_spec.rb"].sort