Fix matcher with undefined property checks

[albe]

Previously, an object matcher that contained an undefined property would not match documents that contained that property, but only during the run of the creation. Once the index was reopened, the matcher would work as expected, because the undefined matcher property was not serialized into the index.
This change fixes that by correctly ignoring matcher properties that are undefined (e.g. when deleting a property from an object).

[coveralls]

Coverage increased (+0.003%) to 97.233% when pulling 4c88c01 on undefined-matcher into bb2ab76 on master.