Tidy up a warning

include/boost/beast/http/impl/verb.ipp

@@ -72,6 +72,10 @@ verb_to_string(verb v)
     }
 
     BOOST_THROW_EXCEPTION(std::invalid_argument{"unknown verb"});
+
+    // Help some compilers which don't know the next line is
+    // unreachable, otherwise spurious warnings are generated.
+    return "<unknown>";
 }
 
 template<class = void>