Fix serializer ampersand warning

WHAT?
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I've been seeing this warning:

    oat/lib/oat/serializer.rb:54: warning: `&' interpreted as argument prefix

You can reproduce this by running rspec with the "-w" flag. For some reason I was seeing it every time I ran the tests for an oat serialiser in my own rails app.

HOW?
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Putting parenthesis around the ampersanded-variable removes the ambiguity and removes the warning.