Associative array access with concatenated keys without quotes fails to parse

This valid bash snippet fails to parse correctly.

```
#!/bin/bash
declare -A things=( ["FOO"]=41 ["FOO BAR"]=42 )
declare foo=FOO
declare bar=BAR
echo "${things[$foo]}" # Works
echo "${things["$foo $bar"]}" # Works
echo "${things[$foo $bar]}" # Doesn't work
```
