executeScript changes driver's execution context, breaking any test that involves .switchTo().frame(...)

Steps to reproduce:

1. Switch to a subframe using `driver.switchTo().frame("a")`
2. Cast the driver to a JavascriptExecutor, and call the executeScript
   method.
3. Observe that the called function runs in the context of the top-level
   document instead of the frame identified by "a".
4. Further, observe that any calls to findElement operate on the wrong
   context.

These steps have been implemented in a test case, that offers a potential fix (in the comments) for *users* of the OperaDriver who need to circumvent this bug.

Summarized: Calling `executeScript()` changes the frame context to _top.

Here's an excerpt of the log file:

```
  <testcase classname="com.opera.core.systems.FramesTest" name="switchToFrameAndExecuteScript" time="0.432">
    <failure message="expected:&lt;[a]&gt; but was:&lt;[]&gt;" type="junit.framework.AssertionFailedError">junit.framework.AssertionFailedError: expected:&lt;[a]&gt; but was:&lt;[]&gt;
        at com.opera.core.systems.FramesTest.switchToFrameAndExecuteScript(FramesTest.java:77)
        at com.opera.core.systems.testing.TestRunner.runChild(TestRunner.java:113)
        at com.opera.core.systems.testing.TestRunner.runChild(TestRunner.java:47)
</failure>
  </testcase>
```