Fix a tiny bug with SigningFetcher.php in php shindig

Here are what I did in the patch:

1. there is a bug when signed fetch a get request.
postParams is null and this line of code will report a warning:
$msgParams = array_merge($msgParams, $postParams);

2. added an unittest for signed fetch post request with header "Content-Type:application/json"

3. modified verification process in SigningFetcherTest.php.

[panjie.pan, 2009-04-02 10:23:08]

updated. add base64_encode after sha1