sharedkv: avoid ambiguity for numeric-like string keys
While we only store URLs and binary SHA-1/SHA-256 values in skv
at the moment, we may store potentially ambiguous keys/values in
the future. It's possible to store "02" and have it treated as
`2' unless explicitly binding parameters as SQL_BLOB. This
behavior was independent of the sqlite_unicode parameter as
evidenced by the new tests.
I only noticed this bug while hacking on another project using
DBD::SQLite, and not while hacking on public-inbox itself.