https://github.com/rails/rails/commit/98bf64bcb9648f88bff4cb59a7ae4db2b6410241
introduced a StringScanner to ensure that value is a valid hstore
document.

However, the negative lookbehind in the regex used to find the final
double-quote of both the key and the value (/(?<!\\)(?=")/) doesn't
differentiate between a backslash-escaped double-quote and a
backslash-escaped backslash followed by an unescaped double-quote.

In other words a valid hstore document such as:

    postgres=# select '"\\"=>"\\"'::hstore;
       hstore
    ------------
     "\\"=>"\\"

will be incorrectly deemed invalid.

This commit aims to rectify that by switching from scan_until to scan
and lazily matching zero or more of either:

- an escaped pair (either \\ or \"); or

- a character that doesn't need escaping

until the positive lookahead matches a double-quote.

The tests have been updated to include both a key and value that end in
a backslash.