I'd like to be able to change Encoding.default_encoding for testing  
purposes.  In particular, I'd like to write a unit test that ensures  
RubyGems loads .gemspec files without the magic comment as UTF-8.   
Something like:

   def test_self_load_specification_utf_8
     orig_encoding = Encoding.default_external
     Encoding.default_external = Encoding.find 'ASCII'

     # ... test setup

     pi = "\317\200"
     pi.force_encoding 'UTF-8'

     assert_equal pi,  
Gem::SourceIndex.load_specification(spec_file).author
   ensure
     Encoding.default_external = orig_encoding
   end

Currently the only way to do this is at ruby startup time through the  
LANG env var or -E.  Is it too dangerous to add  
Encoding::default_external= ?