Fixed small issues revealed in tests

- Default argument for gethostbyname missing
- Fixed invalid return value for close on unconnected socket

Author: geky

ESP8266Interface.cpp

@@ -210,10 +210,11 @@ int ESP8266Interface::socket_close(void *handle)
     int err = 0;
     _esp.setTimeout(ESP8266_MISC_TIMEOUT);
 
-    if (!_esp.close(socket->id)) {
+    if (socket->connected && !_esp.close(socket->id)) {
         err = NSAPI_ERROR_DEVICE_ERROR;
     }
 
+    socket->connected = false;
     _ids[socket->id] = false;
     delete socket;
     return err;

ESP8266Interface.h

@@ -70,7 +70,7 @@ class ESP8266Interface : public NetworkStack, public WiFiInterface
      *                  version is chosen by the stack (defaults to NSAPI_UNSPEC)
      *  @return         0 on success, negative error code on failure
      */
-    virtual nsapi_error_t gethostbyname(const char *name, SocketAddress *address, nsapi_version_t version);
+    virtual nsapi_error_t gethostbyname(const char *name, SocketAddress *address, nsapi_version_t version = NSAPI_UNSPEC);
 
     /** Set the WiFi network credentials
      *