This fixes a bug in which passed-in values files were not correctly
merged into the chart's default values YAML data. I believe it also
fixes some other prioritization bugs in values merging.

The existing unit test was wrong (see TestCoalesceValues). It is
fixed now. Also added more tests to simulate issue #971.

In the course of writing this, I removed some vestigial code as
mentioned in #920.

Closes #971
Closes #920