Use more specific test for ::Encoding
Reading ASCII-incompatible files requires b mode

This allows RDoc to properly detect if a file is a binary file or not if
the first line happens to match a supported encoding.

Part of #288
Correctly transcode ASCII-incompatible files

This is not a complete fix as RDoc will still need to be told the
encoding of the input file through a "coding: " directive.  This does
allow RDoc to generate documentation for
https://github.com/ruby/ruby/blob/v0_7_1/doc/irb/irb-tools.rd.jp.

Fixes #288