fix Encoding::UndefinedConversionError

[S-YOU]

If the source code or comments has unicode characters, (and may be in some ruby versions),
rcov_result.encoding results in #Encoding:ASCII-8BIT, but OS encoding is UTF-8, and
conversion happens before file write, and which in turn results in error.

~/.rbenv/versions/2.5.0/lib/ruby/gems/2.5.0/gems/simplecov-rcov-0.2.3/lib/simplecov-rcov.rb:53:in `write': "\xE4" from ASCII-8BIT to UTF-8 (Encoding::UndefinedConversionError)

[vincentjiang]

@S-YOU
I meet this problem too.

gems/simplecov-rcov-0.2.3/lib/simplecov-rcov.rb:52:in `write': "\xE8" from ASCII-8BIT to UTF-8 (Encoding::UndefinedConversionError)

[brightball]

Same problem.

[vincentjiang]

@fguillen Please review and fix the bug.