[Rd] format bug and patch

[Aleksey Vorona]

Dear R-team,

I've been using R for a while and decided to contribute some bug
fixes. The first bug I tried to solve was
https://bugs.r-project.org/bugzilla3/show_bug.cgi?id=15411

I have attached a patch with the fix to the bug and would love to hear
comments about its quality.

Also, while testing this bug I found another related issue:
> format(complex(real=10, imaginary=4), digits = 1);
[1] "10+0i"

I think this should've been "10+4i". I have entered this as a bug
#15427. But a patch for formatComplex() would be a bigger change, than
the patch for formatReal() I made. So, before I start, I would like to
gauge your opinion.

Do you agree it is a bug?