fix(partiallyDereference): shallow copy node

[Jerska]

I have no clue why, but when pulling the latest version of swagger-markdown, the build fails with this error:

TypeError: Cannot convert undefined or null to object
    at Function.entries (<anonymous>)
    at partiallyDereference (xxx/node_modules/swagger-markdown/bin/convert.js:47:26)
    at partiallyDereference (xxx/node_modules/swagger-markdown/bin/convert.js:43:20)
    at xxx/node_modules/swagger-markdown/bin/convert.js:38:18
    at Array.map (<anonymous>)
    at partiallyDereference (xxx/node_modules/swagger-markdown/bin/convert.js:37:26)
    at xxx/node_modules/swagger-markdown/bin/convert.js:38:18
    at Array.map (<anonymous>)
    at partiallyDereference (xxx/node_modules/swagger-markdown/bin/convert.js:37:26)
    at partiallyDereference (xxx/node_modules/swagger-markdown/bin/convert.js:43:20)

The issue is on Object.entries(node), but logging it shows that it's neither null or undefined, it's the complete API. I don't know why, but it seems like swaggerParser.api is not a regular object, all I know is that adding this line before the call to Object.entries fixed the issue:

node = Object.assign({}, node);

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