to_rational_coeffs: return None if all numerical coeffs are Rationals

    Do not return something if numeric coefficients are already
    Rationals or else a recursion error at Line 974 of polyroots will
    ensue.

    Formerly, the presence of a sqrt(non-number) was giving a return
    value which lead to the recursion error under certain conditions.
add note to nsolve and watch for spurious 'method' keyword
edit docstring; remove obsolete comment; add coverage tests
ellipse: handle prec=None case and arbitrary ellipse failure
_tsolve: return None for all unhandled expressions
6393: RootOf evaluation uses bisection for real roots
unrad: remove unused "all" keyword (use minpoly instead)
6393,6451: watch for RootOf.evalf interval collapse

At low precision, the interval may collapse before a
successful starting point is found for the secant method.
When the interval collapses, that value is returned for
root.
quick