[FIX] sprurious error in log during javascript unit tests

During some tests, the runner performs sequences of DUPLICATE and DROP
on databases. Each DUPLICATE is followed by some operations then a
DROP.

Problem is, after a DROP the next DUPLICATE RPC query will attempt a
connection to the just-dropped database, openerp.sql_db will log an
OperationalError ("Connection to the database failed" since the db it
tries to connect to was removed) then the HTTP dispatcher will just
switch to "nodb" and continue on its merry way.

Mute the sql_db logger while attempting to fetch the ir.http object.

Also assert that the logger name is a string in mute_logger, as
`mute_logger(openerp.sql_db)` is syntactically valid, but will not do
anything.