Prevent prototype corruption when writing object property where
a prototype object has a getter property of the same name.

See https://sourceforge.net/p/htmlunit/bugs/1633/
Test case for prototype corruption bug fixed in last commit.
See https://sourceforge.net/p/htmlunit/bugs/1633/
Code style consistency.
Remove test case (will be added to htmlunit-core-js instead).
Merge pull request #3 from jakecobb/master

Prevent prototype corruption when writing object property.