Fix Ruby 2.7 compatibility in GenericService.underscore

src/ruby/lib/grpc/generic/service.rb

@@ -31,6 +31,7 @@ module GRPC
     #
     # @param s [String] the string to be converted.
     def self.underscore(s)
+      s = +s # Avoid mutating the argument, as it might be frozen.
       s.gsub!(/([A-Z]+)([A-Z][a-z])/, '\1_\2')
       s.gsub!(/([a-z\d])([A-Z])/, '\1_\2')
       s.tr!('-', '_')

src/ruby/spec/generic/service_spec.rb

@@ -55,6 +55,8 @@ describe GenericService do
       expect(GenericService.underscore('AMethod')).to eq('a_method')
       expect(GenericService.underscore('PrintHTML')).to eq('print_html')
       expect(GenericService.underscore('SeeHTMLBooks')).to eq('see_html_books')
+
+      expect(GenericService.underscore('SeeHTMLBooks'.freeze)).to eq('see_html_books')
     end
   end