Workaround struct inspect bug on JRuby 9.2.0.0

Due to a bug in JRuby 9.2.0.0, mutating the result of `Module#to_s`
actually changes the result of other `Module#to_s` calls to the same
module or class, rather than returning a new `String`.

This broke `AbstractStruct#pr_underscore` which is used when rendering
`#inspect` for structures.

This was already fixed upstream, but as a workaround, let's dup the
String before we mutate it, so as to not trip anyone using this JRuby
version (and to restore Travis to beautiful green).

Issue jruby/jruby#5229

Author: ivoanjo

lib/concurrent/synchronization/abstract_struct.rb

@@ -117,7 +117,7 @@ def ns_merge(other, &block)
 
       # @!visibility private
       def pr_underscore(clazz)
-        word = clazz.to_s
+        word = clazz.to_s.dup # dup string to workaround JRuby 9.2.0.0 bug https://github.com/jruby/jruby/issues/5229
         word.gsub!(/::/, '/')
         word.gsub!(/([A-Z]+)([A-Z][a-z])/,'\1_\2')
         word.gsub!(/([a-z\d])([A-Z])/,'\1_\2')