Fix String#-@ for strings that cannot be deduplicated.

Author: aardvark179

spec/ruby/core/string/uminus_spec.rb

@@ -41,20 +41,23 @@
 
       (-dynamic).should_not equal("this string is frozen".freeze)
       (-dynamic).should_not equal(-"this string is frozen".freeze)
+      (-dynamic).should == "this string is frozen"
     end
 
     it "does not deduplicate tainted strings" do
       dynamic = %w(this string is frozen).join(' ')
       dynamic.taint
       (-dynamic).should_not equal("this string is frozen".freeze)
       (-dynamic).should_not equal(-"this string is frozen".freeze)
+      (-dynamic).should == "this string is frozen"
     end
 
     it "does not deduplicate strings with additional instance variables" do
       dynamic = %w(this string is frozen).join(' ')
       dynamic.instance_variable_set(:@foo, :bar)
       (-dynamic).should_not equal("this string is frozen".freeze)
       (-dynamic).should_not equal(-"this string is frozen".freeze)
+      (-dynamic).should == "this string is frozen"
     end
   end
 

src/main/ruby/core/string.rb

@@ -1594,6 +1594,8 @@ def -@
     unless str.tainted? || !(str.instance_variables).empty?
       Truffle::Ropes.flatten_rope(str)
       Truffle.invoke_primitive(:string_intern, str)
+    else
+      str
     end
   end