backends: When using --include DB, use this DB name in output

[Napsty]

Hello and thanks for this great plugin!

When using the --include parameter to specify a database, the output still shows DB "postgres". This is confusing.
In the performance data however we can see the correct database (mydb).

ck@pgsql~$ /usr/lib/nagios/plugins/check_postgres.pl --action backends --dbpass secret -H 127.0.0.1 -u monitoring --include mydb
POSTGRES_BACKENDS OK: DB "postgres" (host:127.0.0.1) 50 of 100 connections (50%) | time=0.05s mydb=50;90;95;0;100 

Suggestion: When using a single include database, use this database name in the plugin output.