MDL-77003 javascript: improve string helper JSON detection.

Changes in fbc2732d made the JS template string helper method
too strict in regards to what it considers a valid JSON object.

Co-authored-by: Mike Churchward <[email protected]>

lib/amd/src/templates.js

@@ -558,8 +558,7 @@ function(
         }
 
         // Allow json formatted $a arguments.
-        // Added double quote after left curly bracket to differentiate between string and JSON string.
-        if (param.indexOf('{"') === 0) {
+        if (param.match(/^{\s*"/gm)) {
             // If it can't be parsed then the string is not a JSON format.
             try {
                 const parsedParam = JSON.parse(param);