admin search_fields splitting query

[Waldemar Kornewald]

When you specify a ModelAdmin search_field as exact (with "=" prefix) the user's search query is split() and then multiple iexact filters are applied, one for each word. This behavior doesn't make sense with a CharField because we're doing an iexact match (there's no way two distinct filters can match at the same time, except if we're doing a JOIN). It should just take the whole query and match against that.

The same problem probably also affects search queries (with an "@" prefix). The string should probably also be passed directly to the DB.