ui/table: fix width calculation of multi-line column headers

internal/ui/table/table_test.go

@@ -29,6 +29,21 @@ first column
 ----------------------
 data: first data field
 ----------------------
+`,
+		},
+		{
+			func(t testing.TB) *Table {
+				table := New()
+				table.AddColumn("first\ncolumn", "{{.First}}")
+				table.AddRow(struct{ First string }{"data"})
+				return table
+			},
+			`
+first
+column
+------
+data
+------
 `,
 		},
 		{