Enforce encoding.

This fixes issues in Ruby 2.0.0, such as:

 2) Failure:
test_encode_length(ASN1_Test) [test/test_ber.rb:190]:
<"\x81\x80"> (UTF-8) expected but was
<"\x81\x80"> (ASCII-8BIT).
Merge pull request #17 from voxik/ruby-2.0

Enforce encoding.