Throw error when a directory is opened

[maxwellgerber]

We should throw an error when someone attempts to open a mocked directory. When we try to write to a directory currently, we don't do this properly and it causes a confusing error to be thrown.

Node Version: v8.11.3

const mockFS = require('mock-fs');
const fs = require('fs');

mockFS({
   '/my/dir': {FOO: ''}
});

fs.writeFileSync('/my/dir', 'bar');

Results in

TypeError: item.setContent is not a function
    at Binding.<anonymous> (.../node_modules/mock-fs/lib/binding.js:442:12)
    at maybeCallback (.../node_modules/mock-fs/lib/binding.js:53:17)
    at Binding.open (.../node_modules/mock-fs/lib/binding.js:402:10)
    ...

Because we try to zero out the file content here, but there is no content to modify on a directory.

With this patch the same code results in

Error: EISDIR, illegal operation on a directory '/my/dir'
    at Binding.<anonymous> (.../node_modules/mock-fs/lib/binding.js:464:13)
    at maybeCallback (.../node_modules/mock-fs/lib/binding.js:61:17)
    at Binding.open (.../node_modules/mock-fs/lib/binding.js:424:10)

[tschaub]

Thanks @maxwellgerber. I noticed the same thing when writing tests for #243. I hadn't seen this PR yet and implemented a (less complete) fix there. I'll pull in this change when I get a chance. If you're able to add additional tests, that would be great.

[tschaub]

Thanks for the contribution @maxwellgerber!