Adds Helpful Default Values to CSP (Security Headers) & Grammar Corre…

…ctions (#363)

This is an amendment to #344 (the new "Security Headers" section to W3TC).

Because the Content Security Policy (CSP) section can seem daunting at first for inexperienced users I felt it is important to include default values and placeholder field examples that users can rely on and use for this security feature. It makes for a better experience when using W3TC.

This amendment also fixes some grammatical errors I discovered in the Security Headers section. Sorry.

ConfigKeys.php

@@ -1648,23 +1648,23 @@
     ),
     'browsercache.security.csp.connect' => array(
         'type' => 'string',
-        'default' => ''
+        'default' => '\'self\''
     ),
     'browsercache.security.csp.font' => array(
         'type' => 'string',
-        'default' => ''
+        'default' => '\'self\' fonts.gstatic.com'
     ),
     'browsercache.security.csp.script' => array(
         'type' => 'string',
-        'default' => ''
+        'default' => '\'self\' *.googleapis.com'
     ),
     'browsercache.security.csp.style' => array(
         'type' => 'string',
-        'default' => ''
+        'default' => '\'self\' *.googleapis.com'
     ),
     'browsercache.security.csp.img' => array(
         'type' => 'string',
-        'default' => ''
+        'default' => '\'self\' *.gravatar.com'
     ),
     'browsercache.security.csp.media' => array(
         'type' => 'string',
@@ -1692,7 +1692,7 @@
     ),
     'browsercache.security.csp.default' => array(
         'type' => 'string',
-        'default' => ''
+        'default' => '\'none\''
     ),
 	'browsercache.other.replace' => array(
 		'type' => 'boolean',