When executing MoreLikeThis with multiple fields, it should create a query considering all fieldNames [LUCENE-6869]

When executing MLT with multiple fields, it should
considerem them all

If a document has the same term in multiple fields, the
mlt generates a query considering only the field with
the higher idf. This commit changes the behaviour, to
include in the query all fieldnames.

Eg.:
Last behaviour:
Doc ("fieldName1", "value")
("fieldName2", "value")

Old behaviour generates query: "fieldName1:value"
New Behaviour generates query: "fieldName1:value
fieldName2:value"

Proposed solution:
https://github.com/prosanes/lucene-solr/pull/1/files

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Migrated from [LUCENE-6869](https://issues.apache.org/jira/browse/LUCENE-6869) by Pedro Rosanes (@prosanes)

Start at the MoreLikeThis implementation and its existing query-generation tests, then trace how terms shared across multiple fields are selected. Confirm completion by adding coverage showing that matching terms from every fieldName appear in the generated query, rather than only the field with the higher IDF.

Written by the indexing model from the issue text.