Clear button is not accessible

[vtx-anton-chashchin]

Hello, we discovered that clear button which appears when allowClear property used is not accessible due to 2 reasons:

• button wrapper uses tabindex=-1 which makes it inaccessible through the keyboard using tab/shift+tab
• button wrapper uses span tag which prevent to click it via space or shift when it is on focus

I opened PR to fix this and added tests to check behavior, could you check it please

#63

[afc163]

Fixed in #63