only wrap fill in url css function when url is detected

[json2d]

this lets you use colors with the fill prop:

<Hexagon fill={"red"}/>
<Hexagon fill={"#f00"}/>
<Hexagon fill={"http://foo.bar/some-image.jpg"}/>

[jannikbuschke]

@json2d I think it is actually better to remove the "smartness" of the Hexagon and just expose the api of the underlying component. Ill let this PR open to keep track of this issue, but have no intention to merge. Thanks for your effort, hope you will be ok with a slightly different solution.