Fix stringification of hstores containing backslash

[brendanlong]

When working on the Pgx.Value tests I found that converting an hstore containing a backslash to a string and back lost the backlash. Presumably either the stringification code needs to add an extra backslash or the parsing code needs to not remove it.

Also update test_pgx_value.ml with an hstore containing a backslash so we can go up to 100% coverage in that file.

[kevinqiu]

Is this still valid? I would like to start looking at it. I'll avoid assigning to myself in case I don't actually get started but I'll link any ongoing work and assign once I do.

[brendanlong]

I think this is still a real bug. I'd start by creating a test case to be sure.

[brendanlong]

I think this is the same bug as #40. We should probably add a test for this case specifically when we fix it though.