Add tests for fs.writeFile to increase coverage

[humphd]

We have a few branches in fs.writeFile that aren't being tested: https://codecov.io/gh/filerjs/filer/src/master/src/filesystem/implementation.js#L1850. To fix this, we need a few more tests added:

• test that passes invalid flags to options.flags (i.e., doesn't include w)
• test that passes a number for the data argument to write, and expects to have it written as a string.

[humphd]

I would like to work on this if no one else is already doing it. This is my first bug in Filer, so I might need some help.