pinMode does not handle INPUT_PULLDOWN mode

[kursatu]

This is an NRF5 specific pulldown mode, which is very useful to detect
tri-state (low,high, high-impedence) output from chips (eg. MCP73831)

If you add these lines in wiring_digital.cpp within the pinMode functions switch statement, it fixes this problem.
case INPUT_PULLDOWN :
direction = PIN_INPUT;
pin_mode = PullDown;
break;