The usage string of custom flags (defaults section) can change depending on the presence of a value

[avoskresensky]

The defaults section of the usage string is currently composed by analyzing the return of the String() function of the corresponding Value object. As a result, the usage string generated when the Value is empty can differ from the one generated after the Value is set.

The following two invocations are going to produce different results.
(--customFlag has no default)
./program --help
./program --customFlag=10 --help

This is a real nuisance for us as we use custom flags extensively.
Whenever a user requests --help for a subcommand or makes a mistake in one of the flags, the usage info that they get ends up having some bogus defaults.

I've already submitted a pull request with a test case that demonstrates the problem.
#243