THRIFT-3823: Use 'pre' HTML tag when generating non escaped documentation
Patch: Maxime Mouial <maxime.mouial@scmfrance.fr>

This closes #1009
diff --git a/compiler/cpp/src/thrift/generate/t_html_generator.cc b/compiler/cpp/src/thrift/generate/t_html_generator.cc
index a7fa13c..5d7ff16 100644
--- a/compiler/cpp/src/thrift/generate/t_html_generator.cc
+++ b/compiler/cpp/src/thrift/generate/t_html_generator.cc
@@ -405,7 +405,7 @@
     if (unsafe_) {
       f_out_ << tdoc->get_doc() << "<br/>";
     } else {
-      f_out_ << escape_html(tdoc->get_doc()) << "<br/>";
+      f_out_ << "<pre>" << escape_html(tdoc->get_doc()) << "</pre><br/>";
     }
   }
 }