Do not allow spaces in between array header braces

[mattyhall]

Both

[ [foo]]

and

[[foo] ]

Are currently permitted which are the last two invalid tests which are not passing (i.e. are parsing). We should disallow these

[0x5a4]

@mattyhall I think this has already been fixed and you forgot to close it? If this is the case, dont forget to remove the mention of it in the README.