Fix #144 - Force Content Type to match proxied response.

[jason-fox]

Proposed changes

Content-type is defaulting to text/html in Express unless explicitly set.

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[jason-fox]

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[jason-fox]

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[jason-fox]

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[jason-fox]

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[jason-fox]

@apozohue10 - any chance you could look at this before making an 8.3.1 release?