Query string default field

[jimczi]

Currently any query_string query that use a wildcard field with no matching field is rewritten with the _all field.

For instance:

#creating test doc
PUT testing/t/1
{
  "test": {
    "field_one": "hello",
    "field_two": "world"
  }
}
#searching abc.* (does not exist) -> hit
GET testing/t/_search
{
  "query": {
    "query_string": {
      "fields": [
        "abc.*"
      ],
      "query": "hello"
    }
  }
}

This bug first appeared in 5.0 after the query refactoring and impacts only users that use _all as default field.
Indices created in 6.x will not have this problem since _all is deactivated in this version.

This change fixes this problem by ignoring terms without an explicit field if the requested multi fields are not empty.

[jimczi]

I marked this with the discuss label because I don't know if this is the right thing to do.
Another approach would be to fail the query if the requested wildcard fields do not match any field in the mapping. This would fix queries that use a mix of explicit and non-explicit field like:

"query": "field1:foo AND bar"

[jpountz]

LGTM

[jpountz]

maybe extract settings.fieldsAndWeights() to a variable to avoid this weird indentation

[jimczi]

I discussed with @jpountz and we agreed that we should not simply ignore terms that expand to an empty list of field. Instead we should rewrite the specific clause to a MatchNoDocsQuery.
I pushed e1ca3fa to fix the behavior.