Fixes for failing tests

[SamWheating]

A few of these tests are failing on master currently:

> pytest
...
================================================================= short test summary info ==================================================================
FAILED tests/test_query.py::TestQueryBuilder::test_union_datasource - AssertionError: assert {'dataSource'...ryType': None} == {'dataSource'...ryType': N...
FAILED tests/db/test_cursor.py::CursorTestSuite::test_ssl_client_cert_authentication_with_patch_imported - AssertionError: expected call not found.
FAILED tests/db/test_druid_dialect.py::DruidDialectTestSuite::test_do_ping_success - AssertionError: expected call not found.
============================================================== 3 failed, 124 passed in 0.52s ===============================================================

I have made what I think is the most reasonable correction in each case, either reverting breaking code changes or fixing up tests and left comments on this PR explaining my rationale. Let me know what you think!

[SamWheating]

This is a revert of a change made in this commit, which I think was a breaking change to the generation of queries on union datasources. The PR doesn't offer additional details on this change, but the current code goes against the stated behaviour in the docstring and causes test failures.

It looks like the change in question never made it to a release, so I think it should be safe to just return to the previous behaviour.

[SamWheating]

The header is set to True on line 352:

pydruid/tests/db/test_cursor.py

Line 352 in d480d07

cursor = Cursor(url, header=True, ssl_client_cert="path/to/cert")

So I have to assume that the False here is just a typo 🤷

[SamWheating]

This is a partial revert of this commit, which caused test failures because text("SELECT 1") != "SELECT 1" .

Based on this previous PR, using text() here causes issues for SQLAlchemy and should be avoided.

cc @betodealmeida - do you know why this was changed back to text()?