Fix NameError on @ + UPPERCASE token in Objective-C

[pocke]

I got an error when highlighting @FOO as Objective-C.
This pull request will fix the problem.

$ ruby -Ilib -rrouge -e 'p Rouge.highlight("@FOO", "objective_c", "html")'
Traceback (most recent call last):
        18: from -e:1:in `<main>'
        17: from /path/to/rouge/lib/rouge.rb:32:in `highlight'
        16: from /path/to/rouge/lib/rouge/formatter.rb:46:in `format'
        15: from /path/to/rouge/lib/rouge/formatter.rb:74:in `format'
        14: from /path/to/rouge/lib/rouge/formatters/html.rb:12:in `stream'
        13: from /path/to/rouge/lib/rouge/formatters/html.rb:12:in `each'
        12: from /path/to/rouge/lib/rouge/formatter.rb:58:in `filter_escapes'
        11: from /path/to/rouge/lib/rouge/formatter.rb:58:in `each'
        10: from /path/to/rouge/lib/rouge/lexer.rb:451:in `lex'
         9: from /path/to/rouge/lib/rouge/lexer.rb:461:in `continue_lex'
         8: from /path/to/rouge/lib/rouge/regex_lexer.rb:272:in `stream_tokens'
         7: from /path/to/rouge/lib/rouge/regex_lexer.rb:291:in `step'
         6: from /path/to/rouge/lib/rouge/regex_lexer.rb:291:in `each'
         5: from /path/to/rouge/lib/rouge/regex_lexer.rb:294:in `block in step'
         4: from /path/to/rouge/lib/rouge/regex_lexer.rb:291:in `step'
         3: from /path/to/rouge/lib/rouge/regex_lexer.rb:291:in `each'
         2: from /path/to/rouge/lib/rouge/regex_lexer.rb:309:in `block in step'
         1: from /path/to/rouge/lib/rouge/regex_lexer.rb:309:in `instance_exec'
/path/to/rouge/lib/rouge/lexers/objective_c/common.rb:47:in `block (2 levels) in extended': uninitialized constant Rouge::Lexers::ObjectiveCCommon::Error (NameError)
Did you mean?  IOError
               Errno

I guess this error is related to #1378. Because this PR introduced the base variable.

By the way, I'm not sure I should write test code for this fix.
I guess we can add it to spec/visual/samples/objective_c, but I don't know Objective-C syntax.
And it is highlighted as "Error", so I'm wondering if the visual test should contain it.

Please tell me how to write a test if it's necessary 🙇‍♂️

[pyrmont]

@pocke Thanks for taking the time to track down the bug! And yep, just an example in the visual sample is a good way to test this kind of thing. The fact that it highlights as an error is no problem at all. (I've added such an example and pushed to your repo.)

How does it look to you now?

[pocke]

Thanks for adding the test code!
It looks good to me. 👍

[pyrmont]

@pocke Thanks again for the PR. I should have mentioned that this will go out as part of v3.16.0. That's scheduled for release on Tuesday 11 February :)