CSV Writer: Allow different types in output

Adjust the csvio.Writer so that it can handle records with the same
field names but different types.

Closes #4781

zio/csvio/writer.go

@@ -62,7 +62,7 @@ func (w *Writer) Write(rec *zed.Value) error {
 		if err := w.encoder.Write(hdr); err != nil {
 			return err
 		}
-	} else if rec.Type != w.first {
+	} else if rec.Type != w.first && !fieldsEqual(rec.Fields(), w.first.Fields) {
 		return ErrNotDataFrame
 	}
 	w.strings = w.strings[:0]
@@ -96,3 +96,15 @@ func formatValue(typ zed.Type, bytes zcode.Bytes) string {
 	}
 	return zson.FormatValue(zed.NewValue(typ, bytes))
 }
+
+func fieldsEqual(a, b []zed.Field) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i, f := range a {
+		if f.Name != b[i].Name {
+			return false
+		}
+	}
+	return true
+}

zio/csvio/ztests/different-types.yaml

@@ -0,0 +1,14 @@
+zed: '*'
+
+input: |
+  {a:0}
+  {a:"foo"}
+  {a:127.0.0.1}
+
+output-flags: -f csv
+
+output: |
+  a
+  0
+  foo
+  127.0.0.1