Disable b64 for SDJ data on POST requests from browsers by default

In POST requests this usually results in a smaller payload size, and is easier to debug. The node tracker shares this setting across multiple emitters that may not all use POST so we can't make this assumption there.

libraries/browser-tracker-core/src/tracker/index.ts

@@ -187,7 +187,7 @@ export function Tracker(
 
     let // Tracker core
       core = trackerCore({
-        base64: trackerConfiguration.encodeBase64,
+        base64: trackerConfiguration.encodeBase64 ?? trackerConfiguration.eventMethod !== 'post',
         corePlugins: browserPlugins,
         callback: sendRequest,
       }),