remove quotation check for key when decoding map

we don't need to check if the key is surrounded by quotation.
In fact, the key might not be strings if we register an extension to
customize the map key encoder/decoder.It may be an integer, float, or
even a struct.

reflect_map.go

@@ -167,10 +167,6 @@ func (decoder *mapDecoder) Decode(ptr unsafe.Pointer, iter *Iterator) {
 	if c == '}' {
 		return
 	}
-	if c != '"' {
-		iter.ReportError("ReadMapCB", `expect " after }, but found `+string([]byte{c}))
-		return
-	}
 	iter.unreadByte()
 	key := decoder.keyType.UnsafeNew()
 	decoder.keyDecoder.Decode(key, iter)