Add test coverage to CSS class generation (#2285)

Add test coverage to CSS class generation (#2285)

The code that generates CSS is based on a lot of boolean conditions.
The possible combinations of these grows exponentially as we add more
conditions.

Since most of the code is conditional on a single boolean, we tested the
following:

1. All `false`
2. All `true`
3. Each individual flag set to `true`

The methods tested are:

* `StreamEntriesHelper#style_classes`
* `StreamEntriesHelper#microformats_classes`
* `StreamEntriesHelper#microformats_h_class`