[TESTS] Fix NPE in FreqTermsEnumTests.
Tests: Remove HttpClient to only use one Http client

The HTTP client implementation used by the Elasticsearch REST tests is
backed by apache http client instead of a self written helper class,
that uses HttpUrlConnection. This commit removes the old simple HttpClient
class and uses the more powerful and reliable one for all tests.

It also fixes a minor bug, that when sending a 301 redirect, a Location
header needs to be added as well, which was uncovered by the switching
to the new client.

Closes #7003
[DOCS] correct documentation about groovy/mvel defaults and deprecations
CORS: Support regular expressions for origin to match against

This commit adds regular expression support for the allow-origin
header depending on the value of the request `Origin` header.

The existing HttpRequestBuilder is also extended to support the
OPTIONS HTTP method.

Relates #5601
Closes #6891
Docs: Added new entry for the SIREn plugin.

Closes #6961
Docs: Changing ES_MAX_MEM default from '1gb' to '1g'

If you set ES_HEAP_SIZE to '1gb' as suggested, Java will yield an "Invalid initial heap size".

Closes #6824