Background doesn't support changing fit

[verbeeksteven]

Describe the bug
Setting a imgixParams fit to anything but crop has no effect. It still is set to crop as the fit is hardcoded. It should be able to be override by passing a imgixParams fit value.

To Reproduce
https://codesandbox.io/s/540nzkv4qn

Steps to reproduce the behaviour:

1. Create a new Background element
2. Set imgixParams.fit to anything but crop
3. See URL has fit=crop

Expected behaviour
Fit should change depending what is set in imgixParams

Information:

• react-imgix version: latest