Fix: LiteralParser __contains__ method compares value of item with Literal arguments

[mikeweltevrede]

Closes #67.

Instead of checking that type(item) is self.base_type (like is implemented in the base class AbstractParser), we need to check if the item is in the values of the given Literal.

[rmmr]

Would be nice to see this fix merged :)

[mikeweltevrede]

@rmmr I was planning to post a reminder today, great minds think alike :')

[mikeweltevrede]

@rnag Would you have time to look at this PR? Thanks!

[mikeweltevrede]

@rnag Hello :) Have you been able to consider this? Thanks :)