Fixes error with 'not' query against string field.

[Ignigena]

Fixes an error that crashes Mongo when a "not" query is run against a field of type "string".

In the following query an error is thrown due to the modifier variable being passed to self.parseValue() not being correct:

Media.find().where({ type: { 'not': 'folder' } }).exec(function (err, results) { ... });

This results in the query being sent to Mongo as follows:

{ '$ne': /^folder$/i }

Which is invalid and crashes the server with the error MongoError: invalid regular expression operator

[particlebanana]

Thanks!