Should not use ASCII NUL to pad between string array elements.

[attipaci]

The FITS standard is quite clear that an ASCII array column (rA type) is undefined after the FITS ASCII NUL (0x00) byte. Prior to 1.18, we however padded string array components (substrings in the column) with that value, which violates the standard. According to a strict reading of the standard, this rendered everything after the first string component invalid. We should therefore switch to padding string array elements with blank space (ASCII 0x20), since trailing spaces are anyway considered insignificant, and are stripped by most (if not all) tools, including earlier versions of us.

This issue is resolved in PR #442

[attipaci]

Merged with PR #442.