fix(PrettyPrint): jasmine.createSpyObj('obj', ['toString']);

The PrettyPrint handler for scalars uses toString() but if that method
has been spyed on, then the stringify fails with
TypeError: Cannot read property 'length' of undefined.

By hanlding this case earlier in format() we avoid the error.

Add unit test covering this case.

src/core/PrettyPrinter.js

@@ -30,6 +30,8 @@ getJasmineRequireObj().pp = function(j$) {
         this.emitString(value);
       } else if (j$.isSpy(value)) {
         this.emitScalar('spy on ' + value.and.identity);
+      } else if (j$.isSpy(value.toString)) {
+        this.emitScalar('spy on ' + value.toString.and.identity);
       } else if (value instanceof RegExp) {
         this.emitScalar(value.toString());
       } else if (typeof value === 'function') {