Bug#1111513: lintian should not warn about missing recommended-field Priority

[Gioele Barabucci <gioele@svario.it>]

Package: lintian
Version: 2.122.0

Dear lintian maintainers,

dpkg 1.22.12/1.22.13 made the Priority: field optional. It default to 
"optional" if missing.

lintian should thus stop raising a `recommended-field` warning when a 
source package does not include a Priority: field.

Example of such a warning:

    W: bats source: recommended-field (in section for source) Priority 
[debian/control:1]

Regards,

--
Gioele Barabucci