Fix limit setting and add ServerImp tests (RIPD-1463,1458):

Add more test coverage for ServerHandlerImp.cpp. Ensure limit parameter
is propagated from parsed object to in-memory config.

Release Notes
-------------

This fixes a bug whereby the limit parameter on a port configuration was
ignored.

src/ripple/rpc/impl/ServerHandlerImp.cpp

@@ -587,7 +587,7 @@ ServerHandlerImp::processRequest (Port const& port,
         return;
     }
 
-    if (! method)
+    if (method.isNull())
     {
         usage.charge(Resource::feeInvalidRPC);
         HTTPReply (400, "Null method", output, rpcJ);
@@ -822,6 +822,7 @@ to_Port(ParsedPort const& parsed, std::ostream& log)
     p.ssl_ciphers = parsed.ssl_ciphers;
     p.pmd_options = parsed.pmd_options;
     p.ws_queue_limit = parsed.ws_queue_limit;
+    p.limit = parsed.limit;
 
     return p;
 }