Convert tag names to lowercase for comparisons in JS.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | No additional failures, see note below
| Fixed tickets | #105
| License       | MIT
| Doc PR        | -

### Notes

I couldn't get the tests to run with the version of Mink specified in the composer.json. I switched it to @dev and had to comment out line 115 in Selenium2DriverTest, ```parent::testWindowMaximize();```. When I did this, I got the following results for Firefox.

```
Tests: 41, Assertions: 181, Incomplete: 1, Skipped: 2.
```
Incomplete: testMouseEvents
Skipped: testPatternGetWindowNames, testGetWindowName... skipped because ```The "getWindowName method is not available for this session. Skipping this test.```

I got the same results when I apply the fix, so I would say the tests pass.