Ensure mongodb credentials are always sent upon (re)connection

If existing socket is reset, perhaps due to mongod failure,
credentials hash isn't reset. This causes them to not be sent upon
reconnection. Changed to reset at connect() instead of relying on a
disconnect() that may or may not happen.

Also cache last set of credentials so they can be reused when connection
loss is detected by with_connection().

lib/moped/authenticatable.rb

@@ -28,6 +28,7 @@ def apply_credentials(logins)
             login(database, username, password)
           end
         end
+        @original_credentials = credentials.dup
       end
       self
     end

lib/moped/connection.rb

@@ -46,6 +46,7 @@ def alive?
     #
     # @since 1.0.0
     def connect
+      credentials.clear
       @sock = if !!options[:ssl]
         Socket::SSL.connect(host, port, timeout)
       else
@@ -74,7 +75,6 @@ def connected?
     #
     # @since 1.0.0
     def disconnect
-      credentials.clear
       @sock.close
     rescue
     ensure
@@ -217,7 +217,10 @@ def read_data(socket, length)
     #
     # @since 1.3.0
     def with_connection
-      connect if @sock.nil? || !@sock.alive?
+      if @sock.nil? || !@sock.alive?
+        connect
+        apply_credentials(@original_credentials || {})
+      end
       yield @sock
     end
   end