String#format fixed to behave consistently with %

String#format method fixes named placeholders missing in String#%
for rubies < 1.9. Before the change format required curly
bracket placeholders to specify sprintf flags.
E.g. "Name: %{name}s"
This wasn't consistent with implementation in %. This patch
unifies the behaviour.
E.g. "Name: %{name}"

See ruby 1.9 documentation for details:
http://www.ruby-doc.org/core-1.9.3/String.html#method-i-25
http://www.ruby-doc.org/core-1.9.3/Kernel.html#method-i-sprintf