Fixes to rfc7230:

fix #1435, fix #1438

* Example and test can be built on msvc v141 15.9.6
  using /std:c++17 and BOOST_BEAST_USE_STD_STRING_VIEW.

* changed string_view.to_string() to std:string(string_view),
  awaiting availability of Library Fundamentals TS here.

* Reactivated relevant tests to param_list. #ifdef 0 test
  exhibited same assertion failed error in debug mode.
  Now fixed in DEBUG on msvc v141 15.9.6 with
  BOOST_BEAST_USE_STD_STRING_VIEW and /std:c++17.

* Looked up http paramters (transfer-encoding, etc.)
  and changed tests as well as fixing comment to
  match specs.

test/beast/http/chunk_encode.cpp

@@ -195,11 +195,11 @@ public:
                 std::string s;
                 for(auto const& v : ce)
                 {
-                    s.append(v.first.to_string());
+                    s.append(std::string(v.first));
                     s.push_back(',');
                     if(! v.second.empty())
                     {
-                        s.append(v.second.to_string());
+                        s.append(std::string(v.second));
                         s.push_back(',');
                     }
                 }