[osh] Allow ${a[@]:} in addition to ${a[@]: }

This makes our parser shorter and more consistent.

bash and zsh have a quirk where they don't allow the first, without a
space.

osh/word_parse.py

@@ -264,19 +264,19 @@ def _ReadVarOpArg2(self, arg_lex_mode, eof_type, empty_ok):
     def _ReadSliceVarOp(self):
         # type: () -> suffix_op.Slice
         """
+        Looking token after first ':'
+
         ArithExpr? (':' ArithExpr? )? '}'
         """
-        # Looking token after first ':'
-        self._SetNext(lex_mode_e.Arith)
-        self._GetToken()
+        self._NextNonSpace()
 
         cur_id = self.token_type
 
-        begin = arith_expr.EmptyZero  # type: arith_expr_t
-        if cur_id != Id.Arith_Colon:  # A pun for Id.VOp2_Colon
+        if cur_id in (Id.Arith_RBrace, Id.Arith_Colon):  #  ${a:} or ${a::}
+            begin = arith_expr.EmptyZero  # type: arith_expr_t
+        else:
             begin = self.a_parser.Parse()
-            cur_id = self.a_parser.CurrentId()
-        #log('after begin %s', Id_str(cur_id))
+            cur_id = self.a_parser.CurrentId()  # advance
 
         if cur_id == Id.Arith_RBrace:  #  ${a:1} or ${@:1}
             no_length = None  # type: Optional[arith_expr_t]  # No length specified

spec/var-op-slice.test.sh

@@ -350,7 +350,7 @@ argv.py "${s:0:0}"
 ['1', '2', '3']
 ## END
 
-#### Inconsistent - ${array[@]:} is not allowed but ${array[@]: } is
+#### ${array[@]:} vs ${array[@]: }  - bash and zsh inconsistent
 
 $SH -c 'array=(1 2 3); argv.py ${array[@]:}'
 $SH -c 'array=(1 2 3); argv.py space ${array[@]: }'
@@ -363,6 +363,13 @@ $SH -c 's=123; argv.py space ${s: }'
 ['space', '123']
 ## END
 
+## OK osh STDOUT:
+['1', '2', '3']
+['space', '1', '2', '3']
+['123']
+['space', '123']
+## END
+
 ## BUG mksh STDOUT:
 ['space', '123']
 ## END