pulls: make "merged" bool be Option as its not set when listing PRs

[vrutkovs]

What did you implement:

Pull's merged field is made optional. Github's API doesn't set this field when all PRs are listed. It is being set when individual PR is being fetched.

How did you verify your change:

Run examples/pulls.rs - it should output a list of PRs. On current master it throws Error: Codec(Error("missing field merged", line: 1, column: 14689))

What (if anything) would need to be called out in the CHANGELOG for the next release:

Pull's merged field is made optional

[softprops]

Do you know if this is now detected by the presence of one of the other merge fields instead?

[vrutkovs]

It seems merged is not set when PRs are listed, but is set for each individual PR.

Reworked this to make merged be Option.