Implement __nonzero__ for SpooledBytesIO, SpooledStringIO

An empty StringIO instance still has a "truthy" value. In
order to user SpooledBytesIO with the ZipFile library, the
object instance needs to be considered truthy. This patch
implements __nonzero__ so that the truthyness does not fall
back on the underlying __len__ implementation.

Added a test that verifies a SpooledBytesIO is compatible
with the ZipFile library.

boltons/ioutils.py

@@ -180,6 +180,9 @@ class SpooledIOBase(object):
     def __ne__(self, other):
         return not self.__eq__(other)
 
+    def __nonzero__(self):
+        return True
+
 
 class SpooledBytesIO(SpooledIOBase):
     """

tests/test_ioutils.py

@@ -3,6 +3,7 @@ import random
 import string
 import sys
 from unittest import TestCase
+from zipfile import ZipFile, ZIP_DEFLATED
 
 from boltons import ioutils
 
@@ -154,6 +155,13 @@ class BaseTestMixin(object):
         self.assertTrue(self.spooled_flo.isatty() is True or
                         self.spooled_flo.isatty() is False)
 
+    def test_truthy(self):
+        """Make sure empty instances are still considered truthy"""
+        self.spooled_flo.seek(0)
+        self.spooled_flo.truncate()
+        if not self.spooled_flo:
+            raise AssertionError("Instance is not truthy")
+
 
 class TestSpooledBytesIO(TestCase, BaseTestMixin, AssertionsMixin):
     linesep = os.linesep.encode('ascii')
@@ -219,6 +227,15 @@ class TestSpooledBytesIO(TestCase, BaseTestMixin, AssertionsMixin):
         self.spooled_flo.rollover()
         self.assertIsNone(self.spooled_flo.flush())
 
+    def test_zip_compat(self):
+        """Make sure object is compatible with ZipFile library"""
+        self.spooled_flo.seek(0)
+        self.spooled_flo.truncate()
+        doc = ZipFile(self.spooled_flo, 'w', ZIP_DEFLATED)
+        doc.writestr("content.txt", "test")
+        self.assertTrue('content.txt' in doc.namelist())
+        doc.close()
+
 
 class TestSpooledStringIO(TestCase, BaseTestMixin, AssertionsMixin):
     linesep = os.linesep