Add XorT#valueOr

[notxcain]

This PR adds XorT#valueOr which behaves similar to Xor#valueOr.

[adelbertc]

Cool beans, thanks! 👍 once Travis gives green light.

[codecov-io]

Current coverage is 88.85%

Merging #807 into master will increase coverage by +0.01% as of 4d63671

@@            master    #807   diff @@
======================================
  Files          166     166       
  Stmts         2295    2296     +1
  Branches        75      75       
  Methods          0       0       
======================================
+ Hit           2039    2040     +1
  Partial          0       0       
  Missed         256     256       

Review entire Coverage Diff as of 4d63671

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[ceedubs]

👍 looks good. Thanks!