force cache invalidation & forward custom headers

[maorhayoun]

hi, i've added 2 features which we found useful:

1. forward of custom headers from client through the proxy (till now only content-type was persisted and returned from api/cache)
2. force invalidation of cached response, upon usage of query param http://proxyurl?force=true

[coveralls]

Pull Request Test Coverage Report for Build 86

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 88.581%

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Totals
Change from base Build 85:	0.2%
Covered Lines:	170
Relevant Lines:	183

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💛 - Coveralls