Call doWritePrefix() if IOutputFormat works as Processor (i.e. clickhouse-local)

This fixes the *WithNames output format (lack of header before this
patch).

src/Processors/Formats/IOutputFormat.cpp

@@ -46,6 +46,12 @@ IOutputFormat::Status IOutputFormat::prepare()
 
 void IOutputFormat::work()
 {
+    if (!prefix_written)
+    {
+        doWritePrefix();
+        prefix_written = true;
+    }
+
     if (finished && !finalized)
     {
         if (rows_before_limit_counter && rows_before_limit_counter->hasAppliedLimit())

src/Processors/Formats/IOutputFormat.h

@@ -87,6 +87,8 @@ private:
     /// Counters for consumed chunks. Are used for QueryLog.
     size_t result_rows = 0;
     size_t result_bytes = 0;
+
+    bool prefix_written = false;
 };
 }