Fix frozen string errors

Author: DavidS

lib/puppet/parser/functions/fqdn_rand_string.rb

@@ -38,7 +38,7 @@
 
   rand_string = ''
   for current in 1..length # rubocop:disable Style/For : An each loop would not work correctly in this circumstance
-    rand_string << charset[function_fqdn_rand([charset.size, (args + [current.to_s]).join(':')]).to_i]
+    rand_string += charset[function_fqdn_rand([charset.size, (args + [current.to_s]).join(':')]).to_i]
   end
 
   rand_string

spec/functions/validate_x509_rsa_key_pair_spec.rb

@@ -176,7 +176,6 @@ def truncate_middle(string)
     start_pos = middle - (chars_to_truncate / 2)
     end_pos = middle + (chars_to_truncate / 2)
 
-    string[start_pos...end_pos] = ''
-    string
+    string[0..start_pos] + string[end_pos..-1]
   end
 end