FIX: use b'' when escaping array as strings in ps

[tacaswell]

The function quote_ps_string is used to sanitize the result of
np.tostring (which really returns bytes) to make sure no 'special'
ps characters make it through.

We were trying to use unicode to replace in a byte string

We also need to decode bytes to unicode to put the string into the
ps file.

closes #6226

[mdboom]

We also need to decode bytes to unicode to put the string into the
ps file.

That's probably fine to get this working for now, but PS is an 8-bit format (completely non-unicode aware), so ideally we wouldn't return strings from this. This makes it more convenient because the default open works on Python 3, but the user could theoretically end up encoding to some weird unicode encoding in the file this way, when it really should be bytes all the way down.