Expand specs for Float#<=> to catch more specialisation cases

Author: chrisseaton

spec/ruby/core/float/comparison_spec.rb

@@ -14,6 +14,8 @@
 
   it "returns nil when the given argument is not a Float" do
     (1.0 <=> "1").should be_nil
+    (1.0 <=> "1".freeze).should be_nil
+    (1.0 <=> :one).should be_nil
   end
 
   it "compares using #coerce when argument is not a Float" do