Fix: Bootstrap 4 styling - when a collection button was used other th…

…an the right most button, its styling would be off, due to corner radius being used.

- DD-646
- https://datatables.net/forums/discussion/comment/140218#Comment_140218

js/buttons.bootstrap4.js

@@ -53,6 +53,11 @@ $.extend( true, DataTable.Buttons.defaults, {
 				disabled: 'disabled'
 			}
 		}
+	},
+	buttonCreated: function ( config, button ) {
+		return config.buttons ?
+			$('<div class="btn-group"/>').append(button) :
+			button;
 	}
 } );
 

js/dataTables.buttons.js

@@ -641,6 +641,13 @@ $.extend( Buttons.prototype, {
 
 		this._addKey( config );
 
+		// Style integration callback for DOM manipulation
+		// Note that this is _not_ documented. It is currently
+		// for style integration only
+		if( this.c.buttonCreated ) {
+			inserter = this.c.buttonCreated( config, inserter );
+		}
+
 		return {
 			conf:         config,
 			node:         button.get(0),