Added test to reproduce NotSerializableException, sample stack:

Fixed bug by removing unncecesary attempt to serialize a Serializable

Error before fixing the bug:
testSerializableOfParcelableExtra(org.robolectric.shadows.IntentTest)  Time elapsed: 0.015 sec  <<< ERROR!
java.lang.RuntimeException: java.io.NotSerializableException: org.robolectric.shadows.TestParcelable
	at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1183)
	at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:347)
	at java.util.ArrayList.writeObject(ArrayList.java:742)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:988)
	at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1495)
	at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1431)
	at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1177)
	at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:347)
	at org.robolectric.shadows.ShadowIntent.serializeCycle(ShadowIntent.java:646)
	at org.robolectric.shadows.ShadowIntent.putExtra(ShadowIntent.java:263)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at org.robolectric.bytecode.ShadowWrangler$ShadowMethodPlan.run(ShadowWrangler.java:473)
	at android.content.Intent.putExtra(Intent.java)
	at org.robolectric.shadows.IntentTest.testSerializableOfParcelableExtra(IntentTest.java:113)