Fix deduplication in |peg$cleanupExpected|

The deduplication skipped over an expected string right after the one
that was removed because the index variable was incorrectly incremented
in that case.

Based on a patch by @fresheneesz:

  https://github.com/dmajda/pegjs/pull/146

lib/compiler/passes/generate-javascript.js

@@ -880,13 +880,15 @@ module.exports = function(ast, options) {
     '    }',
     '',
     '    function peg$cleanupExpected(expected) {',
-    '      var i;',
+    '      var i = 0;',
     '',
     '      expected.sort();',
     '',
-    '      for (i = 1; i < expected.length; i++) {',
+    '      while (i < expected.length) {',
     '        if (expected[i - 1] === expected[i]) {',
     '          expected.splice(i, 1);',
+    '        } else {',
+    '          i++;',
     '        }',
     '      }',
     '    }',

lib/parser.js

@@ -450,13 +450,15 @@ module.exports = (function() {
     }
 
     function peg$cleanupExpected(expected) {
-      var i;
+      var i = 0;
 
       expected.sort();
 
-      for (i = 1; i < expected.length; i++) {
+      while (i < expected.length) {
         if (expected[i - 1] === expected[i]) {
           expected.splice(i, 1);
+        } else {
+          i++;
         }
       }
     }

spec/generated-parser.spec.js

@@ -833,7 +833,14 @@ describe("generated parser", function() {
         });
 
         it("removes duplicates from expected strings", function() {
-          var parser = PEG.buildParser('start = "a" / "a"', options);
+          /*
+           * There was a bug in the code that manifested only with three
+           * duplicates. This is why the following test uses three choices
+           * instead of seemingly sufficient two.
+           *
+           * See https://github.com/dmajda/pegjs/pull/146.
+           */
+          var parser = PEG.buildParser('start = "a" / "a" / "a"', options);
 
           expect(parser).toFailToParse("b", { expected: ['"a"'] });
         });