handle multibyte string when linting text with redpen (#1416)

* handle multibyte string when linting text with redpen

* fix error when no string is provided, fix test's expect value

* remove ambiguious `==` operator

test/handler/test_redpen_handler.vader

@@ -23,6 +23,15 @@ Execute(redpen handler should handle errors output):
   \     'type': 'W',
   \     'code': 'Spelling',
   \   },
+  \   {
+  \     'lnum': 1,
+  \     'col': 35,
+  \     'end_lnum': 1,
+  \     'end_col': 55,
+  \     'text': 'Found possibly misspelled word "コードチェック".',
+  \     'type': 'W',
+  \     'code': 'Spelling',
+  \   },
   \ ],
   \ ale#handlers#redpen#HandleRedpenOutput(bufnr(''), [
   \   '[',
@@ -50,6 +59,21 @@ Execute(redpen handler should handle errors output):
   \   '        "lineNum": 1,',
   \   '        "sentenceStartColumnNum": 0,',
   \   '        "message": "Found possibly misspelled word \"NeoVim\"."',
+  \   '      },',
+  \   '      {',
+  \   '        "sentence": "ALEはNeoVimとVim8で非同期のコードチェックを実現するプラグインです。",',
+  \   '        "endPosition": {',
+  \   '          "offset": 27,',
+  \   '          "lineNum": 1',
+  \   '        },',
+  \   '        "validator": "Spelling",',
+  \   '        "lineNum": 1,',
+  \   '        "sentenceStartColumnNum": 0,',
+  \   '        "message": "Found possibly misspelled word \"コードチェック\".",',
+  \   '        "startPosition": {',
+  \   '          "offset": 20,',
+  \   '          "lineNum": 1',
+  \   '        }',
   \   '      }',
   \   '    ]',
   \   '  }',