File.dirname: add a spec for Shift JIS handling

[byroot]

While trying to speedup various File.* methods, I realized they were way slower and complicated than they should for no apparent reason. However after asking Nobu he explained that Shift JIS encoded text can contain 0x5C (ASCII backslash) as the second byte of a two byte character sequence.

Since on Windows 0x5C is File::ALT_SEPARATOR, this can easily break naive path related algorithms searching for directory separators.

cc @eregon what do you think of that spec? I think it would have helped me figure things out way sooner, so would have been valuable. If you agree I can generalize it to more "path handling" methods.

[trinistr]

I think this can be made easier to understand:

Suggested change

	valid.reject do |enc|
	path = "/foo/bar".encode(enc)
	expected = "/foo".encode(enc)
	File.dirname(path) == expected
	end.should == []
	invalid.reject do |enc|
	path = "/foo/bar".encode(enc)
	rescue Encoding::ConverterNotFoundError
	true
	else
	begin
	File.dirname(path)
	false
	rescue Encoding::CompatibilityError
	true
	end
	end.should == []
	valid.should.all? do |enc|
	path = "/foo/bar".encode(enc)
	expected = "/foo".encode(enc)
	File.dirname(path) == expected
	end
	invalid.should.none? do |enc|
	path = "/foo/bar".encode(enc)
	File.dirname(path)
	rescue Encoding::ConverterNotFoundError, Encoding::CompatibilityError
	false
	end

[trinistr]

Though I'm just a contributor, not sure how much my review matters.