Fix expression parsing in export default statements

[adams85]

Fixes #416

See also https://tc39.es/ecma262/#prod-ExportDeclaration

[lahma]

The issue's test case seems to pass now. Should the test case be added to test suite too (I'm also fine with adding FluentAssertions as dependency as xUnit asserts are yucky anywa)?

[adams85]

Should the test case be added to test suite too

Absolutely, I just forgot to add it... :) Thanks for pointing this out.

I'm also fine with adding FluentAssertions as dependency as xUnit asserts are yucky anywa

Unfortunately, I'm not familiar with FluentAssertions but I don't think we'd need it for this one. However, feel free to add it if you see it useful.