Handle invalid byte sequences

[jamesstout]

Re #37, I found a file that triggered the error, it was a ._* macOS resource fork.

My proposed fix is to check the encoding:

unless file_contents.valid_encoding?
    puts "Invalid encoding for #{filename_without_extension}" if options[:verbose]
    file_contents = fix_encoding(file_contents)
end

And if invalid, fix it:

def fix_encoding(file_contents)
    file_contents.encode!('UTF-8', 'binary', invalid: :replace, undef: :replace, replace: '')
end

I added my dodgy resource file to the fixtures, it broke the tests [without the new code]. I also added some invalid bytes to unused_class.h to check existing tests pass.

All tests pass. I built the gem and ran against my project with the resource fork, and it worked fine.

[dangerpr-bot]

	2 Warnings
⚠️	[DEPRECATION] check is deprecated. Please use check! instead.
⚠️	Unless you’re refactoring existing code or improving documentation, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#39](https://github.com/dblock/fui/pull/39): Handle invalid byte sequences - [@jamesstout](https://github.com/jamesstout).

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[dblock]

I don't think this is right. Files can have a different encoding and we should support that.