Make fittedImage and croppedImage work for URLs with spaces

[jvoigtlaender]

This issue was first opened (by @srikumarks) as an issue against core: https://github.com/elm-lang/core/issues/221, then addressed in a pull request: https://github.com/elm-lang/core/pull/222.

I have simply copied over to here, the change in one of the two files, as per Evan's comment: https://github.com/elm-lang/core/pull/222#issuecomment-218326529.

[lpil]

Looks good. Would be nice to have this merged :)