Werk #16553: check_httpv2: Lookup password containing whitespace

Previously, the active check "HTTP web service" failed to parse the site's password store when there was at least one stored password containing whitespaces.

This was observable when configuring "Authentication" with a password from password store, and resulted in a CRIT state and an error message like

error: invalid value 'http_pass' for '--auth-pw-pwstore <AUTH_PW_PWSTORE>': Can't parse password store: Unexpected format.

in the service's summary.

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