Incorrect calculation of the slide width when using properties 'fixedWidth' and 'gutter'.

[daryavolakh]

Issue description: The values of these properties are concatenated. For example, with a 'width: 400' and 'gutter: 96' in the inspector I see the width of the slider equals to 40096px not the 400px.

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Demo link/slider setting:
let benefitsSlider = tns({
container: '.benefits__slider',
speed: 300,
items: 1,
slideBy: 1,
fixedWidth: '400',
gutter: '96',
mouseDrag: true,
swipeAngle: false,
controls: true,
controlsContainer: '.benefits__slider-nav',
prevButton: '.benefits__slider-previous',
nextButton: '.benefits__slider-next',
nav: false,
});

Tiny-slider version: 2.9.4
Browser name && version: Google Chrome 107.0.5304.110
OS name && version: Ubuntu 20.04.2 LTS