Adding ability to override the built-in cURL parameters.

Previously, these were hard-coded, but now it is possible to add additional parameters, or to change the hard-coded values.

Some use-cases that I have already encountered (there are no doubt many more):
* The ability to set CURLOPT_STDERR (and to set CURLOPT_VERBOSE to true), to aid debugging.
* The ability to set CURLOPT_SSL_VERIFYPEER to false if you don't have appropriate root CA certificates set up, or to set CURLOPT_CAINFO to an appropriate path if cURL isn't picking this up automatically.

Default behaviour is unchanged.

Author: MarkMaldaba

src/Google/Spreadsheet/DefaultServiceRequest.php

@@ -60,6 +60,13 @@ class DefaultServiceRequest implements ServiceRequestInterface
      */
     protected $userAgent = 'PHP Google Spreadsheet Api';
 
+    /**
+     * Additional cURL parameters, or overrides for default parameters.
+     * 
+     * @var array
+     */
+    protected $curlParams = array();
+
     /**
      * Initializes the service request object.
      * 
@@ -118,6 +125,33 @@ public function setUserAgent($userAgent)
         return $this;
     }
 
+    /**
+     * Get explicitly-set cURL parameters.
+     * Does not include the default parameters - just the parameters explicitly set
+     * by the user.
+     * 
+     * @return array
+     */
+    public function getCurlParams()
+    {
+        return $this->curlParams;
+    }
+    
+    /**
+     * Set optional cURL parameters, which can add to or override the default
+     * parameters used for our web requests.
+     * WARNING: Certain over-rides may prevent the code from working.  Use with care!
+     * 
+     * @param array $curlParams associative array of key value pairs
+     *
+     * @return Google\Spreadsheet\DefaultServiceRequest
+     */
+    public function setCurlParams(array $curlParams)
+    {
+        $this->curlParams = $curlParams;
+        return $this;
+    }
+
     /**
      * Perform a get request
      * 
@@ -204,6 +238,10 @@ protected function initRequest($url, $requestHeaders = array())
             CURLOPT_VERBOSE => false,
         );
 
+        // Need to use array addition, rather than array_merge(), as the array is
+        // indexed by numeric keys which we don't want to end up re-indexing.
+        $curlParams = $this->getCurlParams() + $curlParams;
+
         if(substr($url, 0, 4) !== 'http') {
             $url = $this->serviceUrl . $url;
         }