The "srcSet" prop is inconsistent

[kripod]

While preloading images, the srcSet prop is being treated as a single string, but when rendering, it is treated as an array of strings.

I think it would be better to use single strings instead of arrays to follow the standard notation of srcSets, but in order not to break backwards compatibility, dynamic type checking should be used.

[kripod]

@jossmac I think this should not be closed yet, because single strings still cannot be used instead of string arrays for srcSet.

[neptunian]

this probably should not have been merged, as this merge was pending and would have resolved the issue #175