determine read/write from the number of func arguments

rather than whether the value is null. Prevents problems with for
example:

	$inst->config('setthis', 'some value'); // or the default
	...
    $something = $couldBeNull;
	$inst->config('setthis', $something);

which would be pretty confusing.

Also add the possibility to delete a config key - same use case/example
as above

src/Core/InstanceConfigTrait.php

@@ -67,7 +67,7 @@ public function config($key = null, $value = null) {
 			$this->_config = $this->_defaultConfig;
 		}
 
-		if ($value !== null || is_array($key)) {
+		if (is_array($key) || func_num_args() === 2) {
 			return $this->_configWrite($key, $value);
 		}
 
@@ -120,14 +120,19 @@ protected function _configWrite($key, $value = null) {
 			return;
 		}
 
+		if ($value === null) {
+			return $this->_configDelete($key);
+		}
+
 		if (strpos($key, '.') === false) {
 			$this->_config[$key] = $value;
 			return;
 		}
 
 		$update =& $this->_config;
+		$stack = explode('.', $key);
 
-		foreach (explode('.', $key) as $k) {
+		foreach ($stack as $k) {
 			if (!is_array($update)) {
 				throw new Error\Exception(sprintf('Cannot set %s value', $key));
 			}
@@ -142,4 +147,38 @@ protected function _configWrite($key, $value = null) {
 		$update = $value;
 	}
 
+/**
+ * Delete a single config key
+ *
+ * @param string $key
+ * @return void
+ */
+	protected function _configDelete($key) {
+		if (strpos($key, '.') === false) {
+			unset($this->_config[$key]);
+			return;
+		}
+
+		$update =& $this->_config;
+		$stack = explode('.', $key);
+		$length = count($stack);
+
+		foreach ($stack as $i => $k) {
+			if (!is_array($update)) {
+				throw new Error\Exception(sprintf('Cannot unset %s value', $key));
+			}
+
+			if (!isset($update[$k])) {
+				break;
+			}
+
+			if ($i === $length - 2) {
+				unset($update[$k]);
+				break;
+			}
+
+			$update =& $update[$k];
+		}
+	}
+
 }

tests/TestCase/Core/InstanceConfigTrait.php

@@ -285,4 +285,59 @@ public function testReadOnlyConfig() {
 		$object->config('throw.me', 'an exception');
 	}
 
+/**
+ * testDeleteSimple
+ *
+ * @return void
+ */
+	public function testDeleteSimple() {
+		$this->object->config('foo', null);
+		$this->assertNull(
+			$this->object->config('foo'),
+			'setting a new key to null should have no effect'
+		);
+
+		$this->object->config('some', null);
+		$this->assertNull(
+			$this->object->config('some'),
+			'should delete the existing value'
+		);
+
+		$this->assertSame(
+			[
+				'a' => ['nested' => 'value'],
+			],
+			$this->object->config(),
+			'deleted keys should not be present'
+		);
+
+	}
+
+/**
+ * testDeleteNested
+ *
+ * @return void
+ */
+	public function testDeleteNested() {
+		$this->object->config('new.foo', null);
+		$this->assertNull(
+			$this->object->config('new.foo'),
+			'setting a new key to null should have no effect'
+		);
+
+		$this->object->config('a.nested', null);
+		$this->assertNull(
+			$this->object->config('a.nested'),
+			'should delete the existing value'
+		);
+
+		$this->assertSame(
+			[
+				'some' => 'string',
+			],
+			$this->object->config(),
+			'deleted keys should not be present'
+		);
+	}
+
 }