fix: Special characters being printed out from recurrent net

fixes issue BrainJS#267

Author: robertleeplummerjr

browser.js

@@ -6,7 +6,7 @@
  *   license: MIT (http://opensource.org/licenses/MIT)
  *   author: Heather Arthur <fayearthur@gmail.com>
  *   homepage: https://github.com/brainjs/brain.js#readme
- *   version: 1.2.6
+ *   version: 1.2.7
  *
  * acorn:
  *   license: MIT (http://opensource.org/licenses/MIT)
@@ -4997,13 +4997,12 @@ var DataFormatter = function () {
     }
   }, {
     key: 'addSpecial',
-    value: function addSpecial() {
-      for (var i = 0; i < arguments.length; i++) {
-        var special = arguments[i];
-        var specialIndex = this.indexTable[special] = this.characters.length;
-        this.characterTable[specialIndex] = special;
-        this.characters.push(special);
-      }
+    value: function addSpecial(special) {
+      var printableValue = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : '';
+
+      var specialIndex = this.indexTable[special] = this.characters.length;
+      this.characterTable[specialIndex] = printableValue;
+      this.characters.push(special);
     }
   }, {
     key: 'toFunctionString',

browser.min.js

@@ -1,7 +1,7 @@
 {
   "name": "brain.js",
   "description": "Neural network library",
-  "version": "1.2.6",
+  "version": "1.2.7",
   "author": "Heather Arthur <fayearthur@gmail.com>",
   "repository": {
     "type": "git",

dist/utilities/data-formatter.js

@@ -165,13 +165,10 @@ export default class DataFormatter {
     return dataFormatter;
   }
 
-  addSpecial() {
-    for (let i = 0; i < arguments.length; i++) {
-      const special = arguments[i];
-      let specialIndex = this.indexTable[special] = this.characters.length;
-      this.characterTable[specialIndex] = special;
-      this.characters.push(special);
-    }
+  addSpecial(special, printableValue = '') {
+    let specialIndex = this.indexTable[special] = this.characters.length;
+    this.characterTable[specialIndex] = printableValue;
+    this.characters.push(special);
   }
 
   toFunctionString() {