Gem::Specification#platform= should reset #cache_file

[luislavena]

At rake-compiler project, we accept a Gem::Specification object and we duplicate it to be able to generate native gems of it.

Problem is, once we dup'ed the gemspec, changing platform do alter the memoized value of cache_file

I know, the eternal cache invalidation ;-)

Steps to reproduce:

spec1 = Gem::Specification.new do |s|
  s.platform = Gem::Platform::RUBY
  # ...
end
puts s1.cache_file

s2 = s1.dup
s2.platform = "x86-mingw32"

puts s2.cache_file

I can't control when the gemspec will be dup'ed as rake-compiler might be loaded later or before the user defined packaging task.