mpercolate REST API should support source parameter

As stated in documentation, we should support `?source=` parameter in mpercolate REST operations.

This is how to reproduce it:

```sh
curl -XDELETE "http://localhost:9200/test"

curl -XPUT "http://localhost:9200/test/.percolator/1" -d'
{
    "query" : {
        "match" : {
            "foo" : "bar"
        }
    }
}'

# This one works
curl -XPOST "http://localhost:9200/test/message/_mpercolate" -d '
{"percolate" : {}}
{"doc" : {"foo" : "bar is in foo"}}
'

# This one gives: BroadcastShardOperationFailedException[[test][2] ]; nested: PercolateException[failed to percolate]; nested: ElasticsearchIllegalArgumentException[Nothing to percolate];
curl -XGET "http://localhost:9200/test/message/_mpercolate?source=%7B%22percolate%22%3A%7B%7D%7D%0A%7B%22doc%22%3A%7B%22foo%22%3A%22bar is in foo%22%7D%7D%0A"
```

Closes #4900.