Support equality filter in db.ListProperty

[eko-wibowo]

db.ListProperty.validate expects the param to be a value to be assigned to the field (which is a List). However the .filter() method expects the value to be a single element. This issue only occurs for db.ListProperty, ndbProperty with repeated flag works just fine.

Without this changes:

python test/mapreduce/input_readers_test.py 
======================================================================
ERROR: testValidate_Filters (__main__.DatastoreInputReaderTest)
Tests validating filters parameter.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test/mapreduce/input_readers_test.py", line 814, in testValidate_Filters
    self.reader_cls.validate(mapper_spec)
  File "/Users/eko/projects/appengine-mapreduce/python/src/mapreduce/input_readers.py", line 640, in validate
    cls._validate_filters(filters, model_class)
  File "/Users/eko/projects/appengine-mapreduce/python/src/mapreduce/input_readers.py", line 683, in _validate_filters
    raise errors.BadReaderParamsError(e)
BadReaderParamsError: Property mvp_str must be a list

With this changes

......................................................................................................WARNING:root:File /testing/file-000 may have been removed. Skipping file.
WARNING:root:File /testing/file-018 may have been removed. Skipping file.
....................................................................
----------------------------------------------------------------------
Ran 170 tests in 26.231s

OK

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[eko-wibowo]

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