Address "literal string will be frozen in the future" warnings

[StalemateInc]

Should resolve #14.

• dup the initial result to avoid freezing and failing down the codepaths that modify it.
• Fix opening the URL by swapping open with URI.open - Kernel monkey-patch has been removed from open-uri long time ago. I did not test it locally with Rubies that predate 2.6.6.