title: delta writestream .option("mergeSchema", "true") issue
body: <p>I have a delta table of 3 columns with data. Now, I have an incoming data with 4 columns so the DF.<strong>writeStream</strong> has to update the data location atleast with 4 columns automatically, so we can recreate the table on the top of the data location. hence the old records will have nulls in the newly added columns, and the recent data will have 4 columns populated</p>
  
  <pre><code>eg:
  id  name   addr             id  name   addr   phone 
  1   lok    UK      ----&gt;     1  lok    UK     null
                               2  ram    US     +1234
  </code></pre>
  
  <p>but while I use the following command as per databricks wesite, it shows,</p>
  
  <pre><code> df.writeStream
        .option("mergeSchema", "true")
        .format("delta")
        .outputMode("append")
        .option("path","/data/")
        .option("checkpointLocation","/checkpoint/")
        .start()
        .awaitTermination()
  
  ERROR: A schema mismatch detected when writing to the Delta table
  To enable schema migration, please set:
  '.option("mergeSchema", "true")'.
  </code></pre>
  
  <p>But I am already using mergeSchema in options. Please advice..
  NOTE: .saveAsTable or .table functions are also not allowed in writeStream</p>