fix(http) Force content type to be 'text/plain'

This will let CORS requests be marked as a simple request and
therefore not require a preflight request.

While Chrome was using text/plain as the content-type, Safari defaults
to application/xml and therefore forcing a non-simple CORS request.

src/loggly.tracker.js

@@ -154,6 +154,7 @@
                 //creating an asynchronous XMLHttpRequest
                 var xmlHttp = new XMLHttpRequest();
 		xmlHttp.open('POST', this.inputUrl, true); //true for asynchronous request
+		xmlHttp.setRequestHeader('Content-Type', 'text/plain');
 		xmlHttp.send(JSON.stringify(data));
 
             } catch (ex) {