allow spy throwError to throw an Object

When using the following code to simulate a node error:

  spyOn(process, 'kill').and.throwError({code: 'ESRCH'})

The object passed in will be converted to a string by the Error
constructor and result in '[object Object]' which is not very useful.
This PR changes the ``throwError`` spy strategy to only convert
strings into an Error object, but any other objects which are passed
in will be thrown as is. This means the spy strategy can never emulate
throwing a bare string ``throw 'error'``, but this would be a backward
incompatible change.

spec/core/SpyStrategySpec.js

@@ -70,7 +70,7 @@ describe('SpyStrategy', function() {
     expect(originalFn).not.toHaveBeenCalled();
   });
 
-  it('allows a non-Error to be thrown, wrapping it into an exception when executed', function() {
+  it('allows a string to be thrown, wrapping it into an exception when executed', function() {
     var originalFn = jasmine.createSpy('original'),
       spyStrategy = new jasmineUnderTest.SpyStrategy({ fn: originalFn });
 
@@ -82,6 +82,18 @@ describe('SpyStrategy', function() {
     expect(originalFn).not.toHaveBeenCalled();
   });
 
+  it('allows a non-Error to be thrown when executed', function() {
+    var originalFn = jasmine.createSpy('original'),
+      spyStrategy = new jasmineUnderTest.SpyStrategy({ fn: originalFn });
+
+    spyStrategy.throwError({ code: 'ESRCH' });
+
+    expect(function() {
+      spyStrategy.exec();
+    }).toThrow({ code: 'ESRCH' });
+    expect(originalFn).not.toHaveBeenCalled();
+  });
+
   it('allows a fake function to be called instead', function() {
     var originalFn = jasmine.createSpy('original'),
       fakeFn = jasmine.createSpy('fake').and.returnValue(67),