GSMNumberDigit: Make this code work on Pharo

* Use WriteStream>>#contents instead of the OrderedCollection>>#asString
* Character eof does not exist.. use any other character for the marking

GSM48.st

@@ -1538,7 +1538,7 @@ Object subclass: GSMNumberDigits [
             digitMap at: 13 put: $a.
             digitMap at: 14 put: $b.
             digitMap at: 15 put: $c.
-            digitMap at: 16 put: Character eof.
+            digitMap at: 16 put: $Z.
             digitMap yourself.
         ].
     ]
@@ -1563,23 +1563,23 @@ Object subclass: GSMNumberDigits [
         | str |
         <category: 'decoding'>
 
-        str := OrderedCollection new.
+        str := WriteStream on: String new.
 
         [aStream atEnd] whileFalse: [
             | in tmp char |
             in := aStream next.
             tmp := in bitAnd: 16r0F.
-            str add: (self mapDigit: tmp).
+            str nextPut: (self mapDigit: tmp).
 
             tmp := (in bitAnd: 16rF0) bitShift: -4.
             char := (self mapDigit: tmp).
-            char = Character eof
+            char = $Z
                 ifFalse: [
-                    str add: char.
+                    str nextPut: char.
             ].
         ].
 
-        ^ str asString
+        ^ str contents
     ]
 
     GSMNumberDigits class >> encodeFrom: aNumber [