[rubygems/rubygems] Reset existing specs when using Bundler::SpecSet#[]=

We have a flaky failure where to equal Bundler specs sneak into the same
SpecSet. This seems like a vector where that could happen so trying this
in case it fixes the flaky.

ruby/rubygems@a33aeb3c4d

Author: deivid-rodriguez

lib/bundler/spec_set.rb

@@ -83,6 +83,8 @@ def [](key)
     end
 
     def []=(key, value)
+      delete_by_name(key)
+
       add_spec(value)
     end