rdoc sometimes failes when it runs on Windows with RUBYOPT=-U environment variable setting.

[dogatana]

rdoc fails when installing rroonga in my environment.

• Windows 10 Pro, Japanese
• ruby 2.3.1p112 (2016-04-26 revision 54768) [i386-mingw32]
• rdoc 4.2.1 (pre-installed) and 5.0.0
• installing rroonga gem (6.1.0 x86-mingw32)

This can be reproduced by running rdoc in rrooga gem directory.

in C:\Programs\ruby\lib\ruby\gems\2.3.0\gems\rroonga-6.1.0-x86-mingw32
> rdoc
uh-oh! RDoc had a problem:
"\x90" followed by "\x00" on Windows-31J

run with --debug for full backtrace

I attach a result of "rdoc --debug"
log.txt

I should have filed an issue here first, but I already sent an PR.
#430

[aycabta]

Please show output with backtrace by --debug option on 6.0.0.beta3.

[aycabta]

OK, I couldn't reproduce this on RDoc 6.0.1 on Windows, I guess this problem is solved by #430.