improve docstring and add test fot to_rgb(<float>)

Even if the docstring was saing it, emphasis that to_rgb/to_rgba do not
accept float as input, but string representation of a float.

As one might be tempted to had float->gray conversion add failing test
in case the functionality is added.

Side fixes :
    change tuple([value]*3) to (value,)*3 for speed

In [18]: %timeit  tuple([0.4]*3)
1000000 loops, best of 3: 505 ns per loop

In [19]: %timeit  (0.4,)*3
10000000 loops, best of 3: 23.5 ns per loop

In [20]:  (0.4,)*3 == tuple([0.4]*3)
Out[20]: True

Closes gh-2609
Merge pull request #2610 from Carreau/close-2609

improve docstring and add test fot to_rgb(<float>)