Use $literal when needed in Mongo translator

[davinov]

A filter like column "foo" equals "bar" will be translated as {$match: {foo: "bar"}}.
Likewise, a filter like column "foo" equals "$10" will be translated as {$match: {foo: "$10"}}. Except that the $10 in mongo will be understood as "the column 10", which probably doesn't exist. We should use the $literal operator where the $ and . signs should be read without any interpretation.