fix(button): exclude solid from getting added to the button in the class

This prevents `button-solid-secondary` from being added when it should
just be `button-secondary`.

references #6202

ionic/components/button/button.ts

@@ -322,7 +322,7 @@ export class Button {
       this._renderer.setElementClass(this._elementRef.nativeElement, this._role + '-' + type.toLowerCase(), assignCssClass);
     }
   }
-  
+
   /**
    * @private
    */
@@ -331,7 +331,7 @@ export class Button {
       // Support array to allow removal of many styles at once.
       let styles = (type instanceof Array ? type : [type]);
       styles.forEach(styleName => {
-        let colorStyle = (styleName !== null && styleName !== 'default' ? styleName.toLowerCase() + '-' : '');
+        let colorStyle = (styleName !== null && styleName !== 'default' && styleName !== 'solid' ? styleName.toLowerCase() + '-' : '');
         this._colors.forEach(colorName => {
           this._setClass(colorStyle + colorName, assignCssClass); // button-secondary, button-clear-secondary
         });