sprintf.c: remove magic numbers

* sprintf.c (rb_str_format): name default float precision and
  remove unused initial value.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47213 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
sprintf.c: rational 'f' format

* sprintf.c (rb_str_format): support rational 'f' format.
  [ruby-core:64382] [Bug #10136]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47214 b2dd03c8-39d4-4d8f-98ff-823fe69b080e