RandomStringGenerator claims to be immutable, but isn't

RandomStringGenerator claims to be immutable in the Javadocs, however it accepts a TextRandomProvider object through the builder pattern. This object may altered by external code, thus breaking the immutability claim of our generator.

A possible solution is to adjust the documentation for TextRandomProvider and require implementations to be immutable. Alternatively, we can relax the documentation in RandomStringGenerator to remove the immutability claim or state that the mutability is linked to the mutability of the random source (when provided).

I think we will have to do the latter, since the former would forbid callers from supplying a UniformRandomProvider instance as suggested in the documentation. Thoughts welcome.