Support for avro.schema.url while writing to hive table

Support for writing to Hive table which uses Avro schema pointed to by avro.schema.url is missing.

I have Hive table with Avro data format. Table is created with query like this:

CREATE TABLE some_table
  PARTITIONED BY (YEAR int, MONTH int, DAY int)
  ROW FORMAT SERDE
        'org.apache.hadoop.hive.serde2.avro.AvroSerDe'
      STORED AS INPUTFORMAT
        'org.apache.hadoop.hive.ql.io.avro.AvroContainerInputFormat'
      OUTPUTFORMAT
        'org.apache.hadoop.hive.ql.io.avro.AvroContainerOutputFormat'
      LOCATION 'hdfs:///user/some_user/some_table'
      TBLPROPERTIES (
        'avro.schema.url'='hdfs:///user/some_user/some_table.avsc'
      )

Please notice that there is `avro.schema.url` and not `avro.schema.literal` property, as we have to keep schemas in separate files for some reasons.
Trying to write to such table results in NPE.

Tried to find workaround for this, but nothing helps. Tried:

• setting df.write.option("avroSchema", avroSchema) with explicit schema in string
• changing TBLPROPERTIES to SERDEPROPERTIES
• replacing explicit detailed SERDE specification with STORED AS AVRO

I found that this can be solved by adding a couple of lines in `org.apache.spark.sql.hive.HiveShim` next to `AvroSerdeUtils.AvroTableProperties.SCHEMA_LITERAL` is referenced.