REST API: fix the acceptance of a value of false in the publicize update connection endpoints

The casting function in the REST API does not interpret `(string|bool)` properly and as such passing in `false` as the value of `external_user_ID` was not working correctly. I'm introducing a `WPCOM_JSON_API::is_falsy` method which is a counterpart to `WPCOM_JSON_API::is_truthy` to determine if a passed in string represents a false value. The update publicize endpoints now use that new method to determine if a string of false was passed in. Finally I've updated the documentation for those endpoints to indicate that only a boolean value of `false` is accepted, and not `true`.

Fixes https://github.com/Automattic/io/issues/203

Merges r111923-wpcom.

Author: Joey Kudish

class.json-api.php

@@ -59,6 +59,17 @@ static function is_truthy( $value ) {
 		return false;
 	}
 
+	static function is_falsy( $value ) {
+		switch ( strtolower( (string) $value ) ) {
+			case '0' :
+			case 'f' :
+			case 'false' :
+				return true;
+		}
+
+		return false;
+	}
+
 	function __construct() {
 		$args = func_get_args();
 		call_user_func_array( array( $this, 'setup_inputs' ), $args );