Closes #167 - Add json() for json responses

* Add json() which returns decoded json from the response body
  if the response body is a valid json else throws an exception

src/Response.php

@@ -127,4 +127,30 @@ public function throw_for_status($allow_redirects = true) {
 			throw new $exception(null, $this);
 		}
 	}
+
+	/**
+	 * Returns json decoded response
+	 *
+	 * @throws Requests_Exception If `$this->body` is not a valid json
+	 * @return array
+	 */
+	public function json($assoc = true, $depth = 512, $options = 0) {
+		static $json_errors = array(
+			JSON_ERROR_DEPTH => 'JSON_ERROR_DEPTH - Maximum stack depth exceeded',
+			JSON_ERROR_STATE_MISMATCH => 'JSON_ERROR_STATE_MISMATCH - Underflow or the modes mismatch',
+			JSON_ERROR_CTRL_CHAR => 'JSON_ERROR_CTRL_CHAR - Unexpected control character found',
+			JSON_ERROR_SYNTAX => 'JSON_ERROR_SYNTAX - Syntax error, malformed JSON',
+			JSON_ERROR_UTF8 => 'JSON_ERROR_UTF8 - Malformed UTF-8 characters, possibly incorrectly encoded'
+		);
+
+		$data = json_decode($this->body, $assoc, $depth, $options);
+
+		if (JSON_ERROR_NONE !== json_last_error()) {
+			$last_error = json_last_error();
+			$error = isset($json_errors[$last_error]) ? $json_errors[$last_error] : 'Unknown error';
+			throw new Requests_Exception('Unable to parse JSON data: ' . $error, 'response.invalid', $this);
+		}
+
+		return $data;
+	}
 }

tests/Response.php

@@ -0,0 +1,29 @@
+<?php
+
+class RequestsTest_Response extends PHPUnit_Framework_TestCase {
+    /**
+     * @expectedException Requests_Exception
+     */
+    public function testInvalidJsonResponse() {
+        $response = new Requests_Response();
+        $response->body = 'Invalid JSON';
+        $response->json();
+    }
+
+    public function testJsonResponse() {
+        $response = new Requests_Response();
+        $response->body = '{"success": false, "error": [], "data": null}';
+        $decodedBody = $response->json();
+
+        $expected = array(
+            'success' => false,
+            'error' => array(),
+            'data' => null
+        );
+
+        foreach($expected as $key => $value)
+        {
+            $this->assertEquals($value, $decodedBody[$key]);
+        }
+    }
+}