fastavro gives a default value for unions with nulls that don't have a default value

[movermeyer]

This is somewhat related to #48. Given this example schema:

protocol test {
  record a {
    int x;
    union {null, int} y;
  }
}

the following should fail:

>>> data = StringIO.StringIO()
>>> fastavro.writer(data, schema, [{'val': {'x': 1}}])

However, it does not, giving a default value of null to a field ('y') without a default value.

This stems from the use of .get(), which returns None if the key is not found.
The field does not have a default value, so datum.get('default') returns None, which is then used as the default value.

This is just mixing up the lack of a value, with a value of null. Instead, the code should check for the field's existence in the datum, and return false if it doesn't exist.

This is related to #37, in that having write_record call validate() on itself would catch this. Although I imagine the overhead might raise some ire, so there are some options.

Some pull requests will be made in a bit.

[movermeyer]

In general, this is a problem with not checking to make sure that the datum contains a value for each field or the field has a default.

In this specific case, it results in data being made up.

[movermeyer]

I like @e-heller's solution better. Solves this problem and shouldn't have noticeable impact on performance.

[tebeka]

0.9.10 should fix this

[movermeyer]

Looks good. Thanks.