Use correct default value for the overwrite_existing parameter.

See https://github.com/rhec/pyelasticsearch/commit/88950a96583e538f9d38f4e356a995819a4739ac#L2L288
Merge pull request #169 from jezdez/patch-1

Use correct default value for the overwrite_existing parameter.
Update CHANGELOG; fix docstring typo
Fix silly typo in CHANGELOG
Fix query_raw -> filter_raw in CHANGELOG
Merge branch 'master' into commands